Cluster-vcf-records

* When clustering VCF records (as used by minos) proper handling of heterozygous genotypes. Previously if `GT` was `N/M`, `N!=M`, and `N` and `M` both non-zero, then the record would not get used. Now both alleles are added to the output.
* Fix to only use alleles that comprise A,C,G,T characters only. Other alleles now ignored, instead of making it to the output and breaking minos. This stops in particular alleles like `<foo>` getting used, which are allowed in the VCF spec.

Bug fix: whitespace in fasta header lines resulted in no calls being output after merging.

* update to use either `vcflib foo` (which is installed by `apt install libvcflib-tool`), or existing separated `vcf*` scripts (which is what gets built when installing vcflib from source). Code now first looks for `vcflib` in `$PATH` and if not found, falls back to looking for `vcfbreakmulti`, `vcfallelicprimitives`, `vcfuniq`.
* remove warning from bitarray about deprecated length funtion

Adds support for reading BCF files

Better at finding and removing duplicated paths when clustering VCF, at the cost of run time. Add option to use multiple CPUs when clustering.

Another fix to remove duplicate allele paths