FEAT: Save CIFTI subcortical volumes

[mgxd]

Address #251

• My preference remains only outputting the CIFTI, and allowing users to derive this with wb_command -cifti-separate <input-cifti> COLUMN -volume-all <output-file>
• This will increase output size, as another copy of the BOLD series will now be present.
  • Perhaps we want to output a mask + xfm that can be applied instead (and fits better with the fit/apply approach we are moving towards)

Note: The ROI is not guaranteed to be in LAS orientation, as the reorienting happens directly in the dtseries creation workflow