Mutabilitiy of simplicial complexes

[Travis Scrimshaw]

Hey everyone,

   On #12587, I've made SimplicialComplex behave like Graph in that add/remove_face() modifies the object itself, but I've added an option to make the object immutable, thus hashable. John Palmieri suggested to making it a purely immutable object since add/remove_face() only is called in doctests.

   My thought process for having SimplicialComplex mutable is when doing something that builds up/down simplicial complexes iteratively, such as (weak) k-decomposiblity, would create many transient objects, and then also consistency with other classes such as Graph (and I believe Sequences and Matrix).

Please let me know how I should proceed.

Thank you,

Travis