The Searchlight class now takes niimg as input

This modification was performed for correct handling of non-isotropic images,
and for consistency with the rest of the API.
Tidied up plot_haxby_searchlight.py

Adapted nisl/tests/test_searchlight.py to the new searchlight API.
Fixed plot_simulated_data.py

Fixed what appeared to be a bug in searchlight (removed two lines
with unknown significance).
Fixed docstrings, code clean up.
Used _apply_mask_fmri() in searchlight

The _apply_mask_fmri() function was used instead of an ad-hoc solution,
to avoid code duplication.