Refactor list_variants by adding to superclass

[philsteinberg]

The two list_variants functions do essentially the same thing in this PR and https://github.com/uclahs-cds/tool-SRC-util/pull/22/files. Don't worry about it right now as we want to get the functionalities in first, but I think there is an opportunity to do some refractoring (like moving this function to the superclass and adding a flag for single-sample or multi-sample mode or something). Please open on issue for this either in this repo or in our project repo, thanks!

Originally posted by @lydiayliu in https://github.com/uclahs-cds/tool-SRC-util/pull/23#discussion_r930445465