Add reader/writer for NIFTIv1 and NIFTIv2 files.

This patch adds support for reading and writing NIFTI files, as well
as full access to the NIFTI header.  Versions 1 and 2 of the NIFTI
header are supported, and the reader can also be used to read Analyze 7.5
files, though the only Analyze header elements that are supported are the
ones that overlap with NIFTI.  Automatic compression and expansion for
gzipped files is also provided whenever the filename ends with a .gz
extension.

Change-Id: Iead3afb1a65d8c01aa52f3ca55c6da2f08703375
Merge topic 'nifti' into master

e7aa61da Add reader/writer for NIFTIv1 and NIFTIv2 files.