Support to_hgvs for protein variants

[theferrit32]

Example:
https://www.ncbi.nlm.nih.gov/clinvar/variation/2274894

id: ga4gh:VA.AIm-GH_iqp_bpcIVzi431fl7z-cQimNN
type: Allele
digest: AIm-GH_iqp_bpcIVzi431fl7z-cQimNN
location:
  id: ga4gh:SL.wgFs8Z2Nk4uD7nq_gFbdYH1FhagXkMhL
  type: SequenceLocation
  digest: wgFs8Z2Nk4uD7nq_gFbdYH1FhagXkMhL
  sequenceReference:
    type: SequenceReference
    refgetAccession: SQ.dvlWjX2CGulwfb2ehmkCFn02ah7tEEVB
  start: 260
  end: 261
state:
  type: LiteralSequenceExpression
  sequence: A

Should come out as NP_060204.1:p.Val261Ala.

For now only support this for single-protein-residue changes.