Add minimum distance option to clhs

[kdaust]

Hi @pierreroudier - Happy New Year! Some of the projects we've been using clhs for recently have required a minimum distance between sample points, so that transects don't overlap. I've implimented this into the C++ version; if the user inputs a two column table of coordinates and a minimum distance, then the algorithm adds a distance penalty to the objective function, as well as probabilistically swapping a point that is too close. It seems to work well, as long as there is enough space in the sample area. I could imagine this being a common use case - do you think it's worth pulling into the package?

[dylanbeaudette]

Great idea! I can think of several use-cases here at USDA/NRCS.

[gcperk]

Hi @pierreroudier and @kdaust . Great work on this package. I was hoping to use the implementation of clhs with the minimum distance contained within this PR. It looks like it has stalled out and wondered if this PR is going to be reviewed and incorporated? I would prefer to point to the main clhs package rather than a fork if possible. Thanks in advance!