make most things optional

[satra]

No description provided.

[codecov]

Codecov Report

Merging #234 into nwb2asset will decrease coverage by 0.25%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           nwb2asset     #234      +/-   ##
=============================================
- Coverage      82.29%   82.03%   -0.26%     
=============================================
  Files             51       51              
  Lines           4333     4276      -57     
=============================================
- Hits            3566     3508      -58     
- Misses           767      768       +1     

Flag	Coverage Δ
#unittests	82.03% <100.00%> (-0.26%)	⬇️

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Impacted Files	Coverage Δ
dandi/models.py	81.10% <100.00%> (ø)
dandi/tests/fixtures.py	95.91% <0.00%> (-2.15%)	⬇️

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[yarikoptic]

Looks good to me, and I will leave honors to @jwodder to merge