ENH: added 'hsvs' option to 5ttgen + mrtransform fix

[teanijarv]

• added Hybrid Surface and Volume Segmentation ('hsvs') option to the 5TT image generation class
• removed the in_file existing requirement (exists=True -> exists=False) for 5ttgen because hsvs requires a directory not a file as input
• changed '-inverse' parameter location from 1 to 2 in mrtransform class (to make it possible to insert it in the command together with other parameters)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.15%. Comparing base (42302ed) to head (7fc44fd). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3630 +/- ## ======================================= Coverage 63.15% 63.15% ======================================= Files 308 308 Lines 40825 40825 Branches 5656 5656 ======================================= Hits 25781 25781 Misses 14031 14031 Partials 1013 1013 ```

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