Give validate command a --schema VERSION option for validating assets and dandiset.yaml

[jwodder]

Closes #375.

[codecov]

Codecov Report

Merging #379 (e45b171) into master (351bd29) will decrease coverage by 0.27%.
The diff coverage is 38.46%.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
- Coverage   83.14%   82.87%   -0.28%     
==========================================
  Files          55       55              
  Lines        5636     5663      +27     
==========================================
+ Hits         4686     4693       +7     
- Misses        950      970      +20     

Flag	Coverage Δ
unittests	82.87% <38.46%> (-0.28%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/validate.py	57.33% <31.42%> (-22.26%)	⬇️
dandi/cli/cmd_validate.py	87.50% <100.00%> (+0.40%)	⬆️
dandi/dandiapi.py	88.47% <0.00%> (+0.92%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 351bd29...e45b171. Read the comment docs.