enhancement: spatSample() and hexagonal grid

[AMBarbosa]

Dear Robert, In this issue you mentioned there was no support for hexagonal tesselation yet. This would actually be quite useful though, and it might be achieved with an added method="hexagonal" option for spatSample() (which could be useful in itself, as it existed in sp::spsample), plus a voronoi() cropped to the limits of the input polygon. Thanks for considering it!