Fmri to dataset

[adkipnis]

Introduces new classes to parse BIDS formatted derivative sub-directories containing fMRI data.
Methods to load and pool SPM-formatted beta coefficients resp. residuals for multiple runs, as well as create descriptors lists.
Methods to save each as files to OS.
Unit tests.

To do:
datasets from such 4d NIfTi files and corresponding CSV

[JasperVanDenBosch]

Here is an overview of what got changed by this pull request:

Issues
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- Added 16
           

Complexity increasing per file
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- pyrsa/data/fmri.py  13
- tests/test_fmri.py  3
         

Clones added
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- pyrsa/data/fmri.py  2
- tests/test_fmri.py  2
         

See the complete overview on Codacy

[JasperVanDenBosch]

hi @adkipnis thanks for your work on this! As there is considerable overlap, I'm merging this with #242 and will continue work on it there, I hope that's ok with you.