Change of datetime type in BaseSearchGetRequest + test improvement

[zstatmanweil]

Related Issue(s): #315

Description:
This closes #315 . I made a minor change to fix that issue of removing Dict as a type option for datetime in BaseSearchGetRequest after a lot of investigation to determine it doesn't ever appear as a Dict downstream. Most of the PR includes expansion of the existing tests.

The current tests weren't catching the issue described in #315 for a couple reasons: 1) some of the tests were testing too many things at once. For example, the temporal tests targeted at a datetime search, also included a bbox or intersects parameter; and 2) in many of the tests, only one item was being added to the test collection and then searched so when a parameter wasn't properly filtering the results (like in this case), it wasn't being caught.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[bitner]

Thanks for all the tests @zstatmanweil ! This looks good to me.