[ENH] - Enable to automagically fill Sources key

[SamGuay]

Following the new id method, I've improved how sidecarChanges work.
Instead of plainly parsing the value of sidecarChanges, it looks for the Sources key and then look for id to swap filename for.

Also possible to pass a plain string to Sources as well, it won't complain that there are no match with id as it happens with the intendedFor key atm.

Very useful when dealing with MP2RAGE data (ie UNIT1 file), see this section of the BIDS spec.

Shall some logging be added to inform the user about the changes happening within the sidecar file?

[codecov-commenter]

Codecov Report

Merging #211 (7b97e42) into master (86f89e3) will decrease coverage by 0.92%.
The diff coverage is 52.94%.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage   80.07%   79.16%   -0.92%     
==========================================
  Files          15       15              
  Lines         833      835       +2     
  Branches      137      138       +1     
==========================================
- Hits          667      661       -6     
- Misses        134      141       +7     
- Partials       32       33       +1     

Flag	Coverage Δ
pytest	79.16% <52.94%> (-0.92%)	⬇️

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

Impacted Files	Coverage Δ
dcm2bids/utils/utils.py	79.31% <ø> (ø)
dcm2bids/acquisition.py	91.66% <52.94%> (-6.64%)	⬇️

[arnaudbore]

What will happen if the key doesn't exist (typo for example) ?
Quick pep8.

[SamGuay]

What will happen if the key doesn't exist (typo for example) ?
If there is a typo (or no id associated with the (faulty) string, it will be parse as a string. So if someone needs to add something else, it will be pass along as a string (or dict, etc.)