Treat '_ses-X*' indicator as "I do not state any session"

[yarikoptic]

Came up in a use case with @Michael-Sun: they have a template exam card for sessions where they have _ses-XX to be changed to corresponding session (they have tens of them! identical) but often RAs, in case of rerunning some scans, just drag that one in addition to already tuned up in scout _ses-. That leads to duplicate session indicator and the reproin puking since it refuses to deal with multiple sessions in the same "scanning session". If we hardcode this session

• pros: would satisfy this use case
• cons: might break workflow for someone with odd session IDs like XX which are valid BIDS

may be : we will add this just as a heuristic if it appears after already some other session indicator was used, although that might be harder to code up.