Add chromosome, start position, end position and reference genome as fields for variants

They should be the first entries in the "extended" set of fields in the following order:

• reference genome: dropdown with:

  • stored value: GRCh38, displayed value: GRCh38 (hg38)

  • stored value: GRCh37, displayed value: GRCh37 (hg19)

  • stored value: NCBI36, displayed value: NCBI36 (hg18)

• chromosome: 1, 2, 3,....22, X, Y

• start position: number

• end position: number

(see also https://github.com/phenotips/phenotips/blob/master/components/patient-data/ui/src/main/resources/PhenoTips/VCF.xml#L87)