C2P refactor

[brryan]

Right now we have multiple fluid C2P implementations and some maybe inelegant code:

1. con2prim_robust has its own success/failure type. This could probably be common to all c2ps
2. c2p initialization can require a lot of arguments. It might be nice to replace this with a struct of c2p_params that can be stashed all at once in params.

So I think either remove the classic c2p (which Im pretty sure is broken anyway) or refactor all our c2p