Cleaned up how the GCM interacts with some diagnostic attributes

The first time ecosys_driver_set_compute_now() is called, it loops through all
of the MARBL diagnostics and sets compute_now = set_in_tavg_contents() for that
diagnostic.

Note that we use set_in_tavg_contents() instead of accumulate_now() because we
need to set compute_now flags before the time manager is set up, and
accumulate_now() returns False for all diagnostics at that stage.