wip
DOC: update docstrings, whitespace in diagnostics
DOC: update docstring for pca

Note parameter is optional
RF: fix travis build for pip change
seems that the spline fitting is the best except for lower slices, need to try lstqsplines with custom knots?
Merge branch 'master' into slice_motion_correction

* master:
  RF: fix travis build for pip change
  DOC: update docstring for pca
  DOC: update docstrings, whitespace in diagnostics
fix fast-fw