Representing intronic variants in VRS

[Mrinal-Thomas-Epic]

When working on representing molecular consequences from VEP in VA-spec (PR 336), we hit some friction while representing intronic variant molecular consequences. We can represent the transcript (using SequenceReference) and molecular consequence terms, but aren't able to represent intronic variant (currently returned as an HGVS by VEP, e.g. ENST00000650946.1:n.439-1599G>C)