ENH: Extract participant ages from BIDS sources

[mgxd]

Closes #62

This PR moves participant age from the global level, instead using it at the single subject, single session workflow level.

Things to think about:

• Keep --age-months command line, or force all input data to require age data in some BIDS file (session/participant)?
• How will we determine the closest matching template cohort for the default output space?

[codecov-commenter]

Codecov Report

Patch coverage: 41.08% and project coverage change: -0.07 ⚠️

Comparison is base (6e2778a) 31.81% compared to head (641c0c3) 31.75%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
- Coverage   31.81%   31.75%   -0.07%     
==========================================
  Files          54       54              
  Lines        4859     4906      +47     
==========================================
+ Hits         1546     1558      +12     
- Misses       3313     3348      +35     

Impacted Files	Coverage Δ
nibabies/cli/run.py	6.66% <ø> (ø)
nibabies/cli/workflow.py	2.73% <0.00%> (+0.07%)	⬆️
nibabies/reports/core.py	19.14% <0.00%> (+1.84%)	⬆️
nibabies/cli/parser.py	2.50% <9.09%> (+0.38%)	⬆️
nibabies/utils/bids.py	28.14% <11.62%> (-8.42%)	⬇️
nibabies/workflows/base.py	26.25% <59.64%> (+14.71%)	⬆️
nibabies/config.py	75.97% <100.00%> (-0.79%)	⬇️
nibabies/tests/test_config.py	100.00% <100.00%> (ø)
nibabies/workflows/bold/base.py	12.94% <100.00%> (+0.05%)	⬆️

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