Add Edge circulators

Adds Edge-Halfedge, Edge-Vertex, and Edge-Face circulators (and corresponding circulator ranges). These are especially useful in combination with smart ranges and their associated functional programming style. For example:

int valence_after_collapse(SmartEdgeHandle _eh)
{
    return _eh.vertices().sum([&](auto _vh) { return _vh.valence(); }) - 2;
}

Their iteration behaviour is defined as follows:

Edge-Halfedge circulator

for (auto heh : eh.halfedges()) {
    // ...
}

is the same as

for (int i = 0; i < 2; ++i) {
    auto heh = eh.halfedge(i);
    // ...
}

Edge-Vertex circulator

for (auto vh : eh.vertices()) {
    // ...
}

is the same as

for (int i = 0; i < 2; ++i) {
    auto vh = eh.vertex(i);
    // ...
}

Edge-Face circulator

for (auto fh : eh.faces()) {
    // ...
}

is the same as

for (int i = 0; i < 2; ++i) {
    auto fh = eh.halfedge(i).face();
    if (fh.is_valid()) {
        // ...
    }
}

Notes

One additional change was adding a default argument to PolyConnectivity::halfedge_handle, i.e.

mesh.halfedge_handle(eh)

is now equivalent to

mesh.halfedge_handle(eh, 0)

This greatly simplifies the implementation of the Generic Circulator, which can now call halfedge_handle() on any center element type to obtain a starting halfedge.