refactor: Separate histogram calculation from PartialSet

[trisyoungs]

This PR splits the histograms for radial distribution function calculation out of PartialSet into their own HistogramSet since this represented a very specific set of data contained in a quite general-use class otherwise.

Required by #2203.

[RobBuchananCompPhys]

Looks good! Just a couple of comments including a suggestion that may/may not be worth implementing.

[RobBuchananCompPhys]

What do you think of a static method in Array2D, flatten, that takes a couple of Array2D<T>'s (like the above full/bound/unboundHistograms_) and flattens their linearArray()s into a single vector?

Suggested change

	for (auto &histo : fullHistograms_.linearArray())
	histo.zeroBins();
	for (auto &histo : boundHistograms_.linearArray())
	histo.zeroBins();
	for (auto &histo : unboundHistograms_.linearArray())
	histo.zeroBins();
	for (auto &histo : Array2D::flatten({fullHistograms_, boundHistograms_, unboundHistograms_}))
	histo.zeroBins();

I believe the helper method could be implemented like:

static std::vector<A> flatten(const std::vector<Array2D<A>> arrays)
{
    std::vector<A> flat;
    for (const auto &a : arrays)
          flat.insert(flat.end(), a.begin(), a.end());
    return flat;
} 

[trisyoungs]

I see what you mean - in fact Adam's zip() operator would allow us to do this in one loop, albeit with three variables. Your suggestion makes a copy of the data, so unfortunately wouldn't have the intended result. I think I will leave this as-is for now, but your suggestion makes me think that some kind of function which takes a lambda and operates on every element in the Array2D would be useful to implement in the future (e.g. fullHistograms_.operate([](auto &histo) { histo.zeroBins(); }))