Add FSL/FLIRT transformation matrix loading support?

[mikebind]

Hello, I recently spent some time figuring out how Slicer transformation matrices are related to the FSL tool FLIRT's transformation matrices (info and discussion here: https://discourse.slicer.org/t/importing-fsl-flirt-transformation-matrices/30819/8?u=mikebind), and @pieper suggested that it might make sense to include this capability in SlicerFreeSurfer. If you agree and can give me some guidance, I'm happy to try to put together a PR with some python code to do the loading. Alternatively, if someone here wants to adapt the code from the gist (e.g. if that would be faster and easier than the time it would take to give me guidance on how to best fit it in with the existing modules), please feel free to use or adapt anything useful from my rough code here https://gist.github.com/mikebind/d116527e29458637602c86b1d4cd8c2c (no licensing restrictions whatsoever). Also, I fully recognize that FSL and FreeSurfer are different tools and it might not make sense to include this functionality at all in SlicerFreeSurfer, I was just following up on the suggestion.