Cleanup GeneSetDb constructor S3 promotion

[lianos]

I converted the GeneSetDb constructor to an S3 generic and exported it in 97e1b5c2bec010a04b321b706329e8f78176cace.

The way it was done was the easiest to implement as it re-uses all the faux-s3 stuff I hamfisted into there anyway, but it's very ugly.

This was done in order to make multiGSEA and its shiny bits more cleanly used in facile.bio