Score HCV mutation combinations

The updates to HCV resistance scores in #436 still don't handle combinations of two or more mutations. @jeff-k has done some exploration of scoring ideas [here][1] and [here][2].

[1]: https://gist.github.com/jeff-k/bcf6b47152fd596ce6c8b6516cc4d731
[2]: https://gist.github.com/jeff-k/4e7cb225a49083c599ac6c76ba6bcd98