Add class methods for creating a Schema from a file or resource

Add method for creating a Schema from a string path, Path object, or
existing Python file object.

Add method for creating a Schema from an lsst-resources ResourcePath
object or a resource string from which the ResourcePath will be
created automatically.

python/felis/datamodel.py

@@ -24,12 +24,16 @@
 from __future__ import annotations
 
 import logging
+import os
 from collections.abc import Sequence
 from enum import StrEnum, auto
-from typing import Annotated, Any, Literal, TypeAlias, TypeVar, Union
+from pathlib import Path
+from typing import IO, Annotated, Any, Literal, TypeAlias, TypeVar, Union
 
+import yaml
 from astropy import units as units  # type: ignore
 from astropy.io.votable import ucd  # type: ignore
+from lsst.resources import ResourcePath
 from pydantic import BaseModel, ConfigDict, Field, ValidationInfo, field_validator, model_validator
 
 from .db.dialects import get_supported_dialects
@@ -253,7 +257,7 @@ def check_units(self) -> Column:
         Raises
         ------
         ValueError
-            Raised If both FITS and IVOA units are provided, or if the unit is
+            Raised if both FITS and IVOA units are provided, or if the unit is
             invalid.
         """
         fits_unit = self.fits_tunit
@@ -1000,3 +1004,54 @@ def get_table_by_column(self, column: Column) -> Table:
             if column in table.columns:
                 return table
         raise ValueError(f"Column '{column.name}' not found in any table")
+
+    @classmethod
+    def from_resource(
+        cls, resource_path: str | ResourcePath, context: dict[str, Any] = {}, validate: bool = True
+    ) -> Schema:
+        """Load a `Schema` from a string representing a ``ResourcePath``.
+
+        Parameters
+        ----------
+        resource_path
+            The ``ResourcePath`` pointing to a YAML file.
+
+        Returns
+        -------
+        `str`
+            The ID of the object.
+        """
+        logger.debug(f"Loading schema from '{resource_path}'")
+        if isinstance(resource_path, str):
+            resource_path = ResourcePath(resource_path)
+        data = resource_path.read()
+        try:
+            yaml_data = yaml.safe_load(data)
+        except yaml.YAMLError as e:
+            raise ValueError(f"Error loading YAML file from '{resource_path}': {e}")
+        return Schema.model_validate(yaml_data, context=context)
+
+    @classmethod
+    def from_file(cls, source: str | Path | IO[str], context: dict[str, Any] = {}) -> Schema:
+        """Load a `Schema` from a file, a ``Path``, or a string representing a
+        file system path.
+
+        Parameters
+        ----------
+        source
+            The file, path, or file object to load the schema from.
+
+        Returns
+        -------
+        `Schema`
+            The schema loaded from the source.
+        """
+        logger.debug("Loading schema from '%s'", source)
+        if isinstance(source, str) or isinstance(source, Path):
+            if not os.path.exists(source):
+                raise FileNotFoundError(f"File '{source}' not found")
+            with open(source) as file:
+                yaml_data = yaml.safe_load(file)
+        else:
+            yaml_data = yaml.safe_load(source)
+        return Schema.model_validate(yaml_data, context=context)