Modify Center_name field of mri_protocol to allow for NULL values

[cmadjar]

Currently, when an mri_protocol is not being restricted for a specify Center_name, users enter either ZZZZ or AAAA and it is understood that an mri_protocol with Center ZZZZ or AAAA is valid for all centres' scans...

Instead of using that weird hack, we should refactor the code and the database table to allow for the Center_name to be NULL. Ideally, instead of having the Center_name column in the mri_protocol, it should be a CenterID column that links back to the CenterID field of the psc table (foreign key).

TODOs:

• modify the default schema to allow NULL values for the Center_name field of the mri_protocol table
• have a patch to modify the Center_name column (might be tricky with the change of center_name into centerID?)
• correct the raisin bread dataset to replace AAAA or ZZZZ by NULL if applicable?
• modify the code in the function in https://github.com/aces/Loris-MRI/blob/main/uploadNeuroDB/NeuroDB/MRI.pm, function identify_scan_db