Finish extending transcript exon model

[ielis]

Finish work on the model.

The main changes include:

• Variant has coordinates, functional annotations, and genotypes.
• use Genotype in the predicates. This is just a refactor, we must still finalize the predicate logic later
• update toy data and set genotypes for the variant @lnrekerle please notice the lines 49 and 54 in the doctest output of tutorial.rst. I think we had a bug here where we would count the same variant with different genotypes twice (e.g. c.123C>T with homalt and het genotypes would be counted as 2 distinct variants despite being just one variant.).
• update signature of FunctionalAnnotator to return a typing.Sequence[TranscriptAnnotation] instead of Variant
• setup VariantTranscriptProteinArtist stub

[lnrekerle]

Looks good!
Let me know if you change the variant_string function in here, or I can in my next PR.

[lnrekerle]

• update toy data and set genotypes for the variant @lnrekerle please notice the lines 49 and 54 in the doctest output of tutorial.rst. I think we had a bug here where we would count the same variant with different genotypes twice (e.g. c.123C>T with homalt and het genotypes would be counted as 2 distinct variants despite being just one variant.).

Just noticed this comment. I can take a look at that after this is merged.