add model for gene fusions

[nlwashington]

we need to add gene fusion modeling to the Genotype and/or Genomic Feature. Use this ticket for discussion. @kshefchek can you post an example of a gene fusion that's in the @monarch-initiative/mckb data. (i can also fetch some from NCBI.)

some considerations to model:

1. make sure to link to both original genes
2. link to any identified fusion-protein products (do these exist?)
3. genomic breakpoint coordinates