Added processing of multi-allelic cases

[PolinaBevad]

Description

This PR adds processing of multi-allelic cases in the somatic mode when several variants are found in one position. Changes are made to somdict() function - added loops through the lists of variants.

Performance has not changed significantly:

Set	Variants count	Multiallelic variants	master	branch
Colo18/Colo19 chr20 with -f 0.001	125799	133	5:04:13	5:05:43