Update datetimes to be tz-aware + bug fixes

[jlmaurer]

Addresses #623

Description

• Python is forcing us to go to timezone-aware datetimes. This turns out to be a huge pain, because datetimes aren't automatically tz-aware (which is probably a good thing), so they have to be assigned. Thankfully, RAiDER assumes everything is in UTC (Zulu time), so it's fairly easy to go thru and fix this. I've updated datetimes throughout to be tz-aware. This required changing how we represent the valid_range variable for the weather model objects, so "Present" is no longer a valid option for time bounds. Instead, the software automatically pulls today in UTC time.
• If you call tropo_delay in Python and you don't explicitly set the grid in the AOI object first, _get_delays_on_cube errors out due to missing the xpts and ypts attributes. xpts and ypts are only intermediate variables, they don't actually matter in terms of your output grid, they always get set properly from the command line, and as long as you set the grid properly prior to called tropo_delay you have no problem. However, this easy addition allows a user to skip setting up the intermediate grid.

Motivation and Context

• Addresses RasterRDR class xpts attribute not set #623
• Fixes timezone-aware issue
• Adds unit tests and a few bug fixes

How Has This Been Tested?

• Added a unit test that replicates the original issue
• All other unit tests pass

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have added an explanation of what your changes do and why you'd like us to include them.
• I have written new tests for your core changes, as applicable.
• I have successfully ran tests with your changes locally.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[jlmaurer]

This is waiting for #629 because GMAO is failing.

[jlmaurer]

This is waiting for #629 because GMAO is failing.

This is ready now!

[bbuzz31]

LGTM once tests pass (ECMWF down?)

[jlmaurer]

There is an issue with dem_stitcher that is causing some of the current test failures, documented here.