support for simplicial filtrations

[corybrunson]

The package should support coercion of simplicial filtrations as well as complexes.

If we include graphs/networks annotated with compatible values, then every data structure included in the current version supports simplicial filtrations. In some cases the structures are different from those for complexes, and some cases the structure includes information on how the filtration was derived.

Most cases should be straightforward. Some exceptions:

• Coercers to graphs/networks will require a new parameter values analogous to index whose character argument gives the name of the attribute that will store the values/weights/heights. Probably no new generics are needed.
• Coercers from graphs will have values receive the name of the attribute from which values are obtained. New generics (e.g. as_rcpp_filtration()) are probably warranted, to avoid behavior that depends on whether this parameter is used. Question: Should the method check that the values respect inclusion? I believe so.
• Coercers to and from TDA-style lists should ensure that $increasing agrees with whether the filtration is sublevel or superlevel. Question: How is this encoded in the simplex tree implementations, or are superlevel filtrations not supported?