Automatically handle cases without anatomicals

[yarikoptic]

In the sample dataset I have, anat only in the ses-1 and the other sessions have no anatomicals.

For the purpose of this quick app I think we have two options and ideally expose them to the user to choose with the first one being default. --missing-anat-session

• no-anat: just do processing without anatomical (simplest, and default)
• skip : skip processing session without error
• use-other: take anatomical from another session if present. If not present in any session - error.