RF: Re-organise MIST Python code

Paul McCarthyrequested to merge
mnt/python into master

This MR re-organises the MIST Python code into a Python package. Previously each MIST python script would hack the sys.path upon execution, adding ${FSLDIR}/python/mist/. The mist_FA_reg script has been converted from bash to Python, and a few small changes have been made so that the code will work with NIfTI files in different formats (.nii, .nii.zst etc).

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