Add database combining to AEPsych utility executable

[mshvartsman]

After #192 and #165 land, the new utility executable should be extended with a new option that can be used to merge databases into a single database.

• The set of databases to be combined can be passed as a glob https://docs.python.org/3/library/glob.html or folder.
• If the databases have non-default experiment or participant IDs, those should be retained as is.
• If participant IDs are default values or missing, the command should warn and exit, except if passed a --reindex argument in which case each db's participant gets a new ID in the combined DB.