Add mapping for HKQuantityTypeIdentifierWalkingSpeed (#44)

# Add mapping for HKQuantityTypeIdentifierWalkingSpeed

## ♻️ Current situation & Problem
The HealthKitOnFHIR library currently does not have a mapping for
[HKQuantityTypeIdentifierWalkingSpeed](https://developer.apple.com/documentation/healthkit/hkquantitytypeidentifierwalkingspeed),
a quantity that represents the average speed of walking between the
start and end timestamps.


## ⚙️ Release Notes 
Adds a mapping for `HKQuantityTypeIdentifierWalkingSpeed`. As LOINC
codes for walking speed also include a fixed time duration over which
the average is taken (e.g. 24 hours), and this quantity does not have a
fixed time period, this quantity cannot be precisely mapped to a LOINC
code.

## 📚 Documentation
The mapping file has been updated and documentation generated
automatically.


## ✅ Testing
A unit test has been added.


### Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md):
- [X] I agree to follow the [Code of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md).

Sources/HealthKitOnFHIR/HealthKitOnFHIR.docc/HKSampleSupportTables.md

@@ -17,5 +17,5 @@ SPDX-License-Identifier: MIT
 - [HKClinicalType](<doc:SupportedHKClinicalTypes>)
     - HealthKitOnFHIR supports 8 of 8 clinical types.
 - [HKQuantityType](<doc:SupportedHKQuantityTypes>)
-    - HealthKitOnFHIR supports 85 of 87 quantity types.
+    - HealthKitOnFHIR supports 86 of 88 quantity types.
 

Sources/HealthKitOnFHIR/HealthKitOnFHIR.docc/SupportedHKQuantityTypes.md

@@ -9,7 +9,7 @@ SPDX-License-Identifier: MIT
              
 -->
 
-HealthKitOnFHIR supports 85 of 87 quantity types.
+HealthKitOnFHIR supports 86 of 88 quantity types.
 
 |HKQuantityType|Supported|Code|Unit|
 |----|----|----|----|
@@ -100,3 +100,4 @@ HealthKitOnFHIR supports 85 of 87 quantity types.
 |[WaistCircumference](https://developer.apple.com/documentation/healthkit/HKQuantityTypeIdentifierWaistCircumference)|✅|[8280-0](http://loinc.org/8280-0)|[in](http://unitsofmeasure.org)|
 |[WalkingAsymmetryPercentage](https://developer.apple.com/documentation/healthkit/HKQuantityTypeIdentifierWalkingAsymmetryPercentage)|✅|[HKQuantityTypeIdentifierWalkingAsymmetryPercentage](http://developer.apple.com/documentation/healthkit)|[%](http://unitsofmeasure.org)|
 |[WalkingHeartRateAverage](https://developer.apple.com/documentation/healthkit/HKQuantityTypeIdentifierWalkingHeartRateAverage)|✅|[HKQuantityTypeIdentifierWalkingHeartRateAverage](http://developer.apple.com/documentation/healthkit)|[beats/minute](http://unitsofmeasure.org)|
+|[WalkingSpeed](https://developer.apple.com/documentation/healthkit/HKQuantityTypeIdentifierWalkingSpeed)|✅|[HKQuantityTypeIdentifierWalkingSpeed](http://developer.apple.com/documentation/healthkit)|[m/s](http://unitsofmeasure.org)|

Sources/HealthKitOnFHIR/Resources/HKSampleMapping.json

@@ -2105,6 +2105,21 @@
                 "system": "http://unitsofmeasure.org",
                 "unit": "beats/minute"
             }
+        },
+        "HKQuantityTypeIdentifierWalkingSpeed": {
+            "codings": [
+                {
+                    "code": "HKQuantityTypeIdentifierWalkingSpeed",
+                    "display": "Walking Speed",
+                    "system": "http://developer.apple.com/documentation/healthkit"
+                }
+            ],
+            "unit": {
+                "code": "m/s",
+                "hkunit": "m/s",
+                "system": "http://unitsofmeasure.org",
+                "unit": "m/s"
+            }
         }
     }
 }

Tests/HealthKitOnFHIRTests/HKQuantitySampleTests.swift

@@ -1561,6 +1561,36 @@ class HKQuantitySampleTests: XCTestCase {
         )
     }
 
+    func testWalkingSpeed() throws {
+        let observation = try createObservationFrom(
+            type: HKQuantityType(.walkingSpeed),
+            quantity: HKQuantity(unit: HKUnit.meter().unitDivided(by: HKUnit.second()), doubleValue: 1.5)
+        )
+
+        XCTAssertEqual(
+            observation.code.coding,
+            [
+                createCoding(
+                    code: "HKQuantityTypeIdentifierWalkingSpeed",
+                    display: "Walking Speed",
+                    system: .apple
+                )
+            ]
+        )
+
+        XCTAssertEqual(
+            observation.value,
+            .quantity(
+                Quantity(
+                    code: "m/s",
+                    system: "http://unitsofmeasure.org".asFHIRURIPrimitive(),
+                    unit: "m/s",
+                    value: 1.5.asFHIRDecimalPrimitive()
+                )
+            )
+        )
+    }
+
     func testHeartRateVariabilitySDNN() throws {
         let observation = try createObservationFrom(
             type: HKQuantityType(.heartRateVariabilitySDNN),
@@ -2408,7 +2438,7 @@ class HKQuantitySampleTests: XCTestCase {
             )
         )
     }
-    
+
     func testDistanceCycling() throws {
         let observation = try createObservationFrom(
             type: HKQuantityType(.distanceCycling),