Add "intergenic" consequence

[holtgrewe]

Is your feature request related to a problem? Please describe. We currently annotate variants on transcripts, intronic, upstream, and downstream. However, we don't annotate intergenic variants.

Describe the solution you'd like Annotate intergenic variants as such intergenic_variant.

Describe alternatives you've considered N/A

Additional context

• http://www.sequenceontology.org/miso/current_svn/term/SO:0001628

[tedil]

The SO term is available and annotated only if there are no overlapping transcripts found, see https://github.com/varfish-org/mehari/blob/bfd396b279393ffc2569424e58c322d9a135e5df/src/annotate/seqvars/csq.rs#L197-L222