improve ISO 8601 parsing for temporal filter

[itcarroll]

The earthaccess package has had a couple issues raised whose resolutions could be enhancements for the python-cmr package.

• H:M:S for Date_To in the Temporal function defaults to 0:0:0 nsidc/earthaccess#190
• dateutil parser breaks search for dates provided in ISO 8601 format by adding timezone offset nsidc/earthaccess#330

I started to implement handling 1) timezone aware datetime objects and 2) ISO-8601 strings that are not specified down to the second over on the earthaccess package. Some of the earthaccess community (@mfisher87, @jhkennedy) suggested we offer our resolution upstream. I have a draft PR (to be linked below) and would appreciate feedback here from both projects.