Make method structs

[oameye]

Instead of method=:total_degree, we should make method structs that can be called like TotalDegree(; kwargs...). This allows to give options to the Homotopy solvers

[jkosata]

This makes sense but now the portion of the code that could be classified as "convenience functions for HomotopyContinuation" is getting pretty big.

In retrospect, I think this would have made sense as a separate small package rather than being entangled with HB.