Support for genetics/pharmacogenetics

Hi,

I'd would love to see support for genetics and pharmacogenetics in the
healthcare schema.

For example, there is no way to even provide metadata for genetics-based
dosing guidelines in MedicalGuideline right now.  An example of such a
guideline can be found here: https://www.pharmgkb.org/guideline/PA166104997

I would like to propose the addition of these types:


   - MedicalEntity
      - GeneticSequence
         - Gene
         - Genome
         - Variant
         - VariantCollection
            - Diplotype
            - Haplotype
            - Genotype


The VariantCollection level is not really necessary, but all it's children
contains a collection of Variants.

I can go into more detail on exactly what I think these types should
be/contain, but wanted to make sure there's consensus on the types that
should be added.


Thanks,
-Mark