GEC-141: Fix process gains when no stations marked as core

This fixes a bug in the process_gains.py script that came up during testing with SKA AA2 simulated data. The error happens since no SKA antennae are marked as being core ("CS"), so the selection of antennae ends up being empty in the normalize_direction function. I've added a conditional that checks for this and skips the core station normalisation if none are selected.