ADAMFlatGenotype is a smaller, flat, simpler version of a genotype schema.

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.
4. Don't ignore VCF files in .gitignore