Update GMM comparisons to use SMT

[cossatot]

This is, for now, relevant to the gmm_demo branch as this work is not yet in master.

There are some functions to calculate ground motions at each station that has a record in the flatfile for any given event. The outer function is here:

hamlet/openquake/hme/model_test_frameworks/gem/gem_test_functions.py

Line 596 in 96c9d27

def predict_gms_for_eq(eq, gm_df, gsim_lt, test_config, imts=(PGA(),)):

There are some functions in the SMT to do very similar comparisons. It would be good to use that code as much as possible to avoid duplications.

@CB-quakemodel your help would be greatly appreciated. We can start with compiling a list of the potential changes before diving into the work, unless it turns out to be quite simple.