Factor out common fields in nwb2asset() and get_default_metadata()

[jwodder]

Closes #456.

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.52%. Comparing base (afae8e8) to head (ba51aad).
Report is 958 commits behind head on master.

Files	Patch %	Lines
dandi/metadata.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
+ Coverage   88.50%   88.52%   +0.01%     
==========================================
  Files          73       73              
  Lines        9306     9295      -11     
==========================================
- Hits         8236     8228       -8     
+ Misses       1070     1067       -3     

Flag	Coverage Δ
unittests	88.52% <97.14%> (+0.01%)	⬆️

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[yarikoptic]

I see no content difference -- why is it jumping like this then -- did we change blacking settings since last time this file was modified?

[jwodder]

No, I just noticed that the signature could fit on one line and felt compelled to fix it.

[yarikoptic]

Thanks! I dislike functions with undocumented side-effects, but that is ok for now. Let's proceed.