BF: delete image instance to avoid windows error

Windows complains when trying to delete the temporary directory ("The process
cannot access the file because it is being used by another process"). Delete
the offending object holding onto the file.
RF: new class for wrapstruct with labels

Refactor adding labels for codes into new wrapstruct class, and adapt
tests.

Allow fields with codes to pass gracefully with 'unknown' when the value
in the encoded field is not in the lookup table.
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.
NF: draft nifti2 support

First pass at 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.
TST: add tests for nifti2 EOL signature

Test nifti2 headers for EOL signature, test the tests.
DOC: typos / small errors in comments / docstrings

Cleanup of nifti1 with spellchecker
RF: remove Freesurfer hack for shape in Nifti2

Freesurfer hack is for long vectors.  No need for this with Nifti2
because of the int64 dimensions.