This [line](https://github.com/pnlbwh/multi-shell-dMRIharmonization/blob/c99736ea65cc207c95798a451c7618ee40440144/lib/debug_fa.py#L95) can have `_FA_ToMNI` just like [this](https://github.com/pnlbwh/dMRIharmonization/blob/ef4de225f4fc287f3802fa297a9ac227729d7886/lib/debug_fa.py#L95) line. We know `_ToMNI` was dropped from multi-shell for conciseness. Should we consider bringing it back?