Inform the user about genome assembly for genotype matrix results, and allow assembly selection

[carolyncaron]

Currently, the genotype matrix can show results for multiple genome assemblies for a single species. Ideally, it would be made clear to the user which assembly version a variant is on, beyond just by looking at the name of the variant.

Additionally, it would make sense to have another dropdown in Step 2 for the user to select their desired genome assembly. I don't know how feasible it would be to allow the chromosomes to be autocompleted if an assembly is selected, but that would also be very useful.

We could also address issue #58 at the same time since we would want to provide guidance for the user on how to select their desired assembly, and by extension, their desired genomic region.