Further tweaks to MGH/MGZ autoalign matrix handling
Further tweaks to MGH/MGZ autoalign matrix handling

- When writing the extracted matrix to a header key-value pair, use a custom formatter in order to compress the matrix data in an identical manner to that used for other matrix key-value pairs (such as the diffusion gradient table).
- Ensure that when reading this matrix data, both new images created using this format (comma-separated, no column alignment) and older images created using older code (space-delimited, possibly space-padded to achieve column alignment) are read correctly.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3