adding AgeReferenceType

[djarecka]

I'm trying to split this older PR from dandi, and I believe I should update the schema before adding my test

[codecov]

Codecov Report

Merging #50 (a681450) into master (c982460) will decrease coverage by 11.62%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master      #50       +/-   ##
===========================================
- Coverage   95.78%   84.16%   -11.63%     
===========================================
  Files          11       11               
  Lines        1021     1023        +2     
===========================================
- Hits          978      861      -117     
- Misses         43      162      +119     

Flag	Coverage Δ
unittests	84.16% <100.00%> (-11.63%)	⬇️

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Impacted Files	Coverage Δ
dandischema/tests/test_models.py	95.23% <ø> (ø)
dandischema/model_types.py	100.00% <100.00%> (ø)
dandischema/models.py	93.83% <100.00%> (+0.01%)	⬆️
dandischema/datacite.py	12.50% <0.00%> (-82.30%)	⬇️
dandischema/tests/test_datacite.py	36.50% <0.00%> (-63.50%)	⬇️

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