[WIP] Support get_class and roundtrip on datetime datasets

[rly]

Motivation

Fix #429. Related to #1312.

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• [ ] Did you update CHANGELOG.md with your changes?
• [ ] Does the PR clearly describe the problem and the solution?
• [ ] Have you reviewed our Contributing Guide?
• [ ] Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov[bot]]

Codecov Report

:x: Patch coverage is 24.32432% with 28 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 37.92%. Comparing base (285fe2b) to head (2e58aa2).

Files with missing lines	Patch %	Lines
src/hdmf/build/classgenerator.py	25.00%	14 Missing and 1 partial :warning:
src/hdmf/build/objectmapper.py	13.33%	13 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (285fe2b) and HEAD (2e58aa2). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (285fe2b)	HEAD (2e58aa2)
	5	1

Additional details and impacted files

@@             Coverage Diff             @@
##              dev    #1313       +/-   ##
===========================================
- Coverage   91.63%   37.92%   -53.71%     
===========================================
  Files          42       42               
  Lines        9613     9637       +24     
  Branches     1941     1948        +7     
===========================================
- Hits         8809     3655     -5154     
- Misses        522     5653     +5131     
- Partials      282      329       +47     

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