[ENH]bedpostx and probtrackx gpu and multithread support

[mauriliogenovese]

This will enable the gpu version of probtrackx2 and add support to specify the number of thread used by bedpostx (by default it uses a thread per available core)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 2.88%. Comparing base (2f85d92) to head (8f89cdc).

Files with missing lines	Patch %	Lines
nipype/interfaces/fsl/dti.py	0.00%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #3722      +/-   ##
=========================================
- Coverage    2.88%   2.88%   -0.01%     
=========================================
  Files        1279    1279              
  Lines       59413   59434      +21     
=========================================
- Hits         1717    1716       -1     
- Misses      57696   57718      +22     

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