enable & expose ground distance power

[corybrunson]

This PR makes minimal changes to the C++ to enable the use of the internal_p argument, which corresponds to the Wasserstein distance parameter designated $q$ in help(wasserstein_distance). It would resolve #47. In addition to C++ and R source code, the documentation and validation vignette are updated accordingly.

While i think external source code should in general be modified only as needed for binding purposes, i think this change is justified by the companion journal article, which states that this parameter is available to users, i.e. the software was originally designed with it in mind. A natural use case is for statistical analysis packages that depend on phutil to be able to tune this parameter.

In a breaking change to the current version, wasserstein_distance() deploys bottleneck_distance() only when p == Inf && q == Inf; when p is large, it proceeds with the internal wassersteinDistance() with a warning. The threshold for this warning is p >= 6, which is smaller than the current p > 20, due to my lesson from experience that R may crash for certain pairs of small diagrams even with these values. We may want to replace the warning with an error, perhaps with a parameter that allows the user to override it; but at least the bottleneck deployment is more honest.

I want to proceed cautiously with this upgrade, but i've experimented enough locally that i feel comfortable initiating it. : )