RF+ENH: save_json - indent=2 by default

[yarikoptic]

• json_dumps_pretty had it set to 2
• many invocations of save_json also had 2 set manually
• embed_nifti has its invocation of json.dumps still set to 3, but when RF: embed_nifti -> embed_dicom_and_nifti_metadata and not invoke it if min_meta #432 is merged, it would just use save_json with defaults, thus go to 2

Altogether, although it would change formatting for some files, I think we better make formatting more consistent sooner (i.e. now) than later.

[codecov]

Codecov Report

Merging #436 into master will increase coverage by 0.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
+ Coverage   74.92%   74.93%   +0.01%     
==========================================
  Files          35       35              
  Lines        2831     2833       +2     
==========================================
+ Hits         2121     2123       +2     
  Misses        710      710              

Impacted Files	Coverage Δ
heudiconv/bids.py	82.97% <66.66%> (ø)
heudiconv/utils.py	90.08% <80.00%> (+0.08%)	⬆️

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[yarikoptic]

just came to my mind the argument "by dcm2niix uses ..." -- apparently it uses TAB for indentation. I guess we just want to make sure to process all .json we get from dcm2niix (either using min_meta or not)

[yarikoptic]

oh well, I think we better make things consistent within a single release (instead of piece meal in multiple), and since we are anyways inconsistent with dcm2niix (or "consistent" with TAB width set to 2 ;)) -- we better proceed by merging.