Support for non-geospatial?

[jeffp123]

I have made a small PR that adds a Supercluster::new_non_geospatial that functions otherwise like a regular Supercluster instance, except it will not apply conversions to/from lat/lng coordinates.

As noted in the PR description, there is still a little bit of updating yet to do, to improve the code comments and testing, but first I would like to first present my approach here and see if there are any comments/suggestions:
https://github.com/chargetrip/supercluster-rs/pull/2/files

[slavik-pastushenko]

Hey @jeffp123!

Thanks for contributing!
I left a few comments, please take a look whenever you can.

[jeffp123]

awesome, thank you! I will update in the next day or so.