[ENH] skullstripping fixes and options

[Remi-Gau]

• Properly ordering entities of output images ; fixes space entities is not always ordered properly after segmentation  #550
• skip skullstrip if input image is skullstripped already
• implement a option opt.skullstrip.do
• add metadata to root of the folder

[codecov]

Codecov Report

Merging #571 (745d740) into dev (1108b2f) will increase coverage by 0.16%.
The diff coverage is 95.34%.

@@            Coverage Diff             @@
##              dev     #571      +/-   ##
==========================================
+ Coverage   72.51%   72.68%   +0.16%     
==========================================
  Files         183      185       +2     
  Lines        4301     4335      +34     
==========================================
+ Hits         3119     3151      +32     
- Misses       1182     1184       +2     

Flag	Coverage Δ
unittests	72.68% <95.34%> (+0.16%)	⬆️

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

Impacted Files	Coverage Δ
src/batches/preproc/setBatchSkullStripping.m	80.00% <92.00%> (+4.13%)	⬆️
src/bids/isSkullstripped.m	100.00% <100.00%> (ø)
src/bids/returnNameSkullstripOutput.m	100.00% <100.00%> (ø)
src/defaults/checkOptions.m	92.66% <100.00%> (+0.06%)	⬆️

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