RF: refactor nifti1 / analyze + tests for nifti2

Put tests into classes and generalize class names and constants ready
for implementing nifti2.
RF: check for overflow in setting hdr dims

The int64 type can overflow when setting large python integers, rather
than silently rolling to negative.
WIP: draft nifti2 support

Need to add algorithm for testing for presence of nifti2 on loading.
Need example data files.  Need to add check for EOL corruption.
NF: add EOL check for new EOL Nifti2 field

Check that field is properly set, raise error otherwise.