"Not null" columns for subclass in single table inheritance
User Story
As a ramsis user, I would like to be able to load seismiccatalogs into the database without error being raised dues to not null columns. Testing ramsis with the new datamodel threw up the following issue: when committing to the seismiccatalog table, starttime should not be null. The change which combines seismiccatalog with seismiccatalogpredictionbin is found in this merge request with single table inheritance, so the startdate and other columns are found in the seismiccatalog table now.
I believe these should be nullable as I read that subclasses in single table inheritance should never be 'not null'. Adding a validation on the orm side to check that the values are populated would preserve functionality.
Acceptance Criteria
ramsis workflow can be run which will load a seismiccatalog into the database and save.