Update variation property to account for multiple alleles

[daisieh]

Addresses issue ga4gh-beacon#57. In order to capture zygosity (and genotype) in CaseLevelVariants completely, we need to be able to account for the situation where a caseLevelVariant contains two alternate alleles, neither of which is the reference. I would recommend requiring the first element of a variations array, element 0, to be the reference allele, and subsequent alternate alleles to be numbered accordingly. Then zygosity can be represented as in the beacon-ri implementation:

            "caseLevelData": [
              {
                "zygosity": {
                  "label": "0/1",
                  "id": "GENO:GENO_0000458"
                },
                "biosampleId": "HG03770"
              }
            ],

with the labeling schema extended in the style of VCF, with values like 1/2.

[daisieh]

More specifically, this allows for the specification of the GENO:0000402 value for zygosity:

compound heterozygous: A heterozygous quality inhering in a single locus complement comprised of two different varaint alleles and no wild type locus. (e.g.fgf8a/fgf8a)