Cluster-vcf-records

* added simple VCF merger
* better clustering where large deletions are involved (with new max_alleles_per_cluster option)
* bug fixes involving recognising that in indel can be represented in more than one way in a VCF file (eg homopolymers)

* new option `merge_method` when clustering. Let's you choose between existing gramtools method, and a simple merging of VCF records, without enumerating through all combinations

* bug fix where some combinations of variants were missed when clustering

Fix typo in vcf_file_read.vcf_file_to_dict(). Option renamed remove_useless_start_nulceotides -> remove_useless_start_nucleotides

New method: VcfRecord. called_alts_from_genotype()

Bug fix key error during clustering

Initial release