Updated N4BiasFieldCorrection

Added weight_image to input spec. Added bspline_order to input spec and
updated arg formatting for bspline_fitting_distance to allow for
bspline_order to be set. Updated n_iterations and convergence_threshold
arg formatting to conform with other arg formatting for this class.
Clean up ants.segmentation

Gave ants.segmentation some PEP8 love. Also updated _run_interface call
to include correct_return_codes which is declared in the overridden
function
Updated examples
Fixed doctest
Updated CHANGES
Forgot to Merge Fork to get other changes
Added PR Link
fix: resolved conflict
Merge pull request #1 from satra/fix/ants

Fix/ants
Merge pull request #990 from blakedewey/master

Update and Clean ants.segmentation
Create CONTRIBUTING.md
fix: made a new example with nipy
doc: updated CHANGES
Merge pull request #992 from akeshavan/restingnipy2

New rsfmri example with nipy