hera_mc/mc_session.py: add special PostgreSQL mode to avoid key errors on ROACH temps
hera_mc/mc_session.py: add special PostgreSQL mode to avoid key errors on ROACH temps

This uses a PostgreSQL-specific syntax to ignore preexisting rows in the ROACH
temperature table. We have to go behind the back of the ORM (object/relational
mapper) a bit, but in this limited use case that seems OK.

HERA-Team/hera_mc,HERA-Team/hera_mc,HERA-Team/Monitor_and_Control