RF - changed memory view to double* trilinear_interpolation_4d

[gabknight]

• Changed the out type from memoryview to double* in dipy.core.interpolation.trilinear_interpolation_4d_c. This allows for the use of trilinear_interpolation4d_c in pure cython. In particular when the out needs to be instanciated in cython. This is relevant for PR WIP - Parallel Tracking Framework #3089
• Replaced dipy.core.interpolation.trilinear_interpolation_4d declaration to def from cpdef

[skoudoro]

Thank you for this @gabknight.

Waiting for the CI's

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.39%. Comparing base (890e49a) to head (fe4bfe6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3090      +/-   ##
==========================================
+ Coverage   82.37%   82.39%   +0.01%     
==========================================
  Files         146      146              
  Lines       20640    20640              
  Branches     3320     3320              
==========================================
+ Hits        17002    17006       +4     
+ Misses       2807     2804       -3     
+ Partials      831      830       -1     

see 1 file with indirect coverage changes

[skoudoro]

LGTM, Thank you @gabknight! merging