Add time varying position field

[jacobagilbert]

One possible implementation, starting point only.

Things I like about this:

• Badly needed, long overdue.
• Can be a single file in a collection OR an aggregated field within a Recording

Things I don't like about this:

• Not sure this belongs in an extension as opposed to
• File cannot be simply appended to with new information
• Definition for time is somewhat weird... geojson would do this with a properties option time but that lives at a higher level than the Point object.
• Several odd edge cases with when time is not included
• Non Conforming Dataset (metadata only)