move `generate_annotation_similarity_graph` switch outside the `calc_similarity`

[rmflight]

Right now generate_annotation_similarity_graph has a switch statement to select which method is actually used for calculating the similarity overlap, and that switch statement is inside the calc_similarity function that is defined locally.

I'm curious if we move that switch statement outside of calc_similarity and instead pass it in as an argument, will we see a speed improvement? Worth trying.