NiftiLabelsMasker can safely process Nifti1Image

In the previous state, passing a Nifti1Image as labels image would modify
it inplace. A copy is now made before modifying it.
Removed unused functions

regions_are_overlapping() and regions_to_mask() have been removed (and
their tests.)
Removed NislImage class.
Added joblib support for NiftiLabelsMasker