ADAMFlatGenotype is a smaller, flat version of a genotype schema

Includes a rewrite of a non-Tribble VCF parser, which parses (large) VCF files line-by-line and includes a converter into the ADAMFlatGenotype format itself.  

Also:
1. Added support for missing genotypes in the VCFLineConverter for ADAMFlatGenotype.
2. Added a ReferenceMapping for the ADAMFlatGenotype
3. the flat genotype converter uses multiple writers for smaller output files.
Yeah, but we can't guarantee we actually get any of that information from the VCF itself!