Allow timestamp indices in queries

[JustinElms]

Priority: low

Is your feature request related to a problem? Please describe.
The Navigator launches with a default dataset configuration that has time = -1. This leads to a number of failed tile queries whenever a user opens the page. This similarly affects users who have to provide a specific timestamp when using the API instead of an index which adds additional steps needed to use the API.

Describe the solution you'd like
I'm sure this has been discuss before but to mitigate this we could add logic that allows the Navigator to accept a timestamp index instead of a specific timestamp value. i.e. -1 would be the most recent timestamp available, 0 the first, etc. This would fix the above problem but also help make the API a little more user friendly. Users could specify an index rather than having to perform a separate query to get the exact timestamp they're looking for.