Use phasing information for comp. het. refinement

[holtgrewe]

Is your feature request related to a problem? Please describe.
GATK writes out phasing information. We don't use this but it would be useful for deciphering two adjacent SNVs if they introduce a stop codon, for example.

Describe the solution you'd like
Collect all variants within a predefined windows (from command line arguments, default 50bp). Write out an output line of all combinations that lie on the same haplotype. It is sufficient to support the GATK PGT FORMAT entry for now.

Describe alternatives you've considered
N/A

Additional context
N/A