Nifti misc improvements

[AnnaBeers]

This pull requests represents a few of QTIM lab's contributions to glue medical, especially pertaining to Nifti file formats and exporting subsets. More to come.

Remaining things to do:

• Create small test NIFTI and NRRD files

[astrofrog]

@AndrewBeers - I had forgotten to commit utils.py, which I've done now - sorry!

[astrofrog]

remove the comma here

[astrofrog]

There are some files that were added here that shouldn't be - specifically all the files in glue_medical.egg-info - can you remove these? By the way, I've just pushed a commit to master to add a .gitignore file (which was missing previously). I also fixed the DICOM tests. Could you rebase this branch?

EDIT: just remembered we had added a .gitignore file here, oops

[astrofrog]

@AndrewBeers - thanks for continuing work on this! Just so you know, since we last worked on this I've now added the ability to write custom subset mask importers/exporters:

http://glueviz.org/en/stable/customizing_guide/customization.html#custom-subset-mask-importers

This is different from the Data/Subset exporter:

http://glueviz.org/en/stable/customizing_guide/customization.html#custom-data-subset-exporters

because in the latter, the values of the subset should be exported, as opposed to the mask.