[WIP] Refactor build/conversion of datetime Data objects

[rly]

Motivation

Fix #1311. Requires #1312. Related to #1313 and might be merged with that.

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]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.97%. Comparing base (285fe2b) to head (a8a4475).

Files with missing lines	Patch %	Lines
src/hdmf/build/objectmapper.py	0.00%	4 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (285fe2b) and HEAD (a8a4475). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (285fe2b)	HEAD (a8a4475)
	5	1

Additional details and impacted files

@@             Coverage Diff             @@
##              dev    #1314       +/-   ##
===========================================
- Coverage   91.63%   37.97%   -53.67%     
===========================================
  Files          42       42               
  Lines        9613     9615        +2     
  Branches     1941     1942        +1     
===========================================
- Hits         8809     3651     -5158     
- Misses        522     5636     +5114     
- Partials      282      328       +46     

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