RF+API: Opener ignores case for compressed exts

Make `Opener` class ignore case when looking for compressed extensions
such as '.gz' or '.bz2', by default.
DOC: docstring for filespec_to_file_map

Add docstring for filespec_to_file_map class method.
RF+TST: allow mixed case image file extensions

Allow image filenames to be mixed case including compressed extensions
like '.gz' and '.bz2'.
RF: allow for images that can't be built in memory

Some images (particularly, PARREC), cannot be easily constructed in
memory, because their headers are rather complicated to construct.

Allow skipping tests that need to construct in-memory images.
NF: add PARREC to top-level load

Allow PAR/REC files to be loaded from top-level `nibabel.load` function.