Suppress a spurious warning by NiftiMasker

A spurious warning was issued in the following case:

   masker = NiftiMasker(mask=mask)
   masker.fit_transform()

i.e. NiftiMasker initialized with a mask then fit_transform() call.
No copy for bool -> int8 conversion by as_ndarray

as_ndarray() does not make a copy when converting from a bool dtype to
a 1-byte dtype.
Tests have been strengthened.
Added fetch_msdl_atlas()

Fetcher for Multi-Subject Dictionary Learning atlas, by the Parietal team,
Neurospin.