Allowing Numericals in JSON custom fields

[smeisler]

Closes #249

[codecov-commenter]

Codecov Report

Merging #250 (1e09113) into master (22a9851) will decrease coverage by 0.40%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage   78.98%   78.59%   -0.40%     
==========================================
  Files          15       15              
  Lines        1009     1009              
  Branches      214      214              
==========================================
- Hits          797      793       -4     
- Misses        165      170       +5     
+ Partials       47       46       -1     

Flag	Coverage Δ
pytest	78.59% <0.00%> (-0.40%)	⬇️

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Files Changed	Coverage Δ
dcm2bids/acquisition.py	94.01% <0.00%> (ø)

... and 1 file with indirect coverage changes