Wrong $REFDIR in mosart’s buildnml

When startup type is “branch” or “hybrid”, the code will look for files in ``$din_loc_root/$run_refdir/$run_refcase/$run_refdate``, in other words, it only accepts ``run_refdir`` as a relative pathname. Moreover, if the files cannot be found in this buildnml’s ``refdir`` and ``get_refcase`` is True, it will kill the run.

Also, if I understand this correctly, when buildnml is called, rpointers and corresponding files should have already been copied/linked to $RUNDIR by check_input_data. Therefore, file check should be performed in $RUNDIR (as in other components’ buildnml).

Removing ``$get_refcase`` and ``$run_refdir`` from mosart’s buildnml solves the problem, in which case files are only checked in $RUNDIR.

Both the perl version (e.g. in maint-1.0) and the python version (master) have this issue.