uniformize the definition of somatic variants

[m-huertasp]

In filter_cohort.py the definition of somatic variants is done by using "VAF", the line is the following:

maf_df_f_somatic = maf_df.loc[maf_df["VAF"] <= somatic_vaf_boundary][["MUT_ID","SAMPLE_ID", "PMEAN", "PSTD"]].reset_index(drop = True)

In other places the definition of somatic is made filtering also for the "vd_VAF" and "VAF_AM" columns. We should uniformize this definition and make sure that it is the same everywhere.