RF: Responded to brikhead.py reviews

Edited brikhead.py in response to @matthew-brett reviews in #561. Cleaned up
doc-strings, added/clarified error mesages to be more specific, and modified
how AFNI files are read in to ensure that they accurately represent underlying
data (i.e., are always 4D). Added `filespec_to_file_map` class method for
`AFNIImage` to separate logic of reading in files with compression; likely need
to edit `filename_parser.types_filenames` instead.

Author: rmarkello