Signals Notebook: Additional fields cannot be added to existing experiments

Description

Signals Notebook prevents adding new experiment attributes retroactively. This is by design. 

An experiment is a snapshot in time of the system template used to create it. The only fields available on it are the ones that were present at its creation time. 

Additionally, fields that are read-only will continue to be read-only even if you change the field properties on the system template. 

Such changes will only affect experiments created after they are saved to the template.