Add support for .nii files

[Abe404]

See #9 (comment)

Andre wants to test the software with .nii files. I think that it's probably easy to add support for this considering we already support compressed nifty (nii.gz).

[andreped]

Made a PR #11.

[Abe404]

I have fixed it I think. See master...nii-support
The branch is: https://github.com/Abe404/RootPainter3D/tree/nii-support

I think it makes sense to test this before merging to master and closing the issue.

[andreped]

Absolutely, please, test that this works before merging :P

Ideally we should have some unit tests for this, but it can be quite tedious to setup for such a huge project. Hence, I would recommend you just testing stuff out yourself, as you normally would.