ENH: Support flexible CIFTI combinations

[tsalo]

Closes #3330. This is ongoing. I am starting with the space parsing (linked with nipreps/niworkflows#883) and will move onto actual processing from there.

Changes proposed in this pull request

• Parse CIFTI --output-spaces options using the pattern <surface_space>:<surface_entity>:<value>::<volume_space>:<volume_entity>:<value>.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 47.27%. Comparing base (fe7c9ff) to head (ae23fc8).

Files	Patch %	Lines
fmriprep/config.py	0.00%	8 Missing ⚠️
fmriprep/workflows/bold/base.py	0.00%	1 Missing ⚠️

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HEAD has 5 uploads less than BASE

Flag	BASE (fe7c9ff)	HEAD (ae23fc8)
	7	2

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3340       +/-   ##
===========================================
- Coverage   71.03%   47.27%   -23.77%     
===========================================
  Files          56       56               
  Lines        4233     4239        +6     
  Branches      638      580       -58     
===========================================
- Hits         3007     2004     -1003     
- Misses       1114     2163     +1049     
+ Partials      112       72       -40     

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