[STORY] Add option to merge two related sessions after staging

[tclose]

Sometimes a single visit is split into two sessions to allow a gap in between to perform other investigations. Would like to have the option to combine these sessions into a single session on XNAT.

Proposal:

Append a suffix to the second or subsquent sessions to the session ID e.g. <SESSION-ID>-2, which is used to modify the scan IDs of the scans to upload, e.g. 10 -> 20010, 602 -> 20602.