Added packing module.

[dwieker]

2D pair correlation function with edge handling.

[danielballan]

Oh, excellent! I have been hoping to get this functionality in trackpy for a long time.

Would you consider putting this pull request against the upstream fork at https://github.com/soft-matter/trackpy where all the contributors can review it?

[danielballan]

P.S. @nkeim I had forgotten that I started watching your fork before we joined forces. :- )

[nkeim]

Hi @danielballan ! I'm actually advising @dwieker on his senior project, so our plan was to discuss the code here before it goes over to the main repo. But your comments are welcome here or anywhere!

[danielballan]

Wonderful. I thought I might be missing some offline conversation here, but I can't resist the opportunity to welcome new potential contributors. Welcome, @dwieker!

[nkeim]

@dwieker This PR should be targeting the "master" branch. (It's confusing because the "numba" branch is the default on my fork, for historical reasons.) But if that proves difficult I can still do some code review here.