feat: Geohash class

[WenJJ2000]

Hi, we (@WenJJ2000 @karlsb @burcukilic @muchembledMartin @LinusWallin) are a group of university students who would like to be assigned this issue. We have worked on the issue and here is our pull request.

Adds Geohash class and Geohash tests which has the following functionalities :

• generating an envelope of the geohash cell
• converting a point to the geohash of given length
• gets the longest geohash that contains the envelope
• and generates up to four geohashes that completely cover given envelope

Solves issue #213

[stolstov]

Thank you for the contribution.

Could you format it using tabs, similar to other files in the repo?

I've done this:

		String geohash1 = "9qh9mzv6sgtkwz34";
		Envelope2D env1 = Geohash.geohashToEnvelope(geohash1);

and got

-38.41176850208285
-38.41176850241027
75.14555980573277
75.14555980556906

This is not what I expect to see. I checked with some other implementation and also with this website http://geohash.co/

		Point2D p = new Point2D(61.57057257930285, -43.79239231349915);
		System.out.println();
		for (int i = 0; i < 12; i++)
			System.out.println(Geohash.toGeohash(p, i + 1));

This prints

m
m2
m25
m25y
m25yc
m25yc5

The values seem to be correct, but it throws at trying to do more than that. I would expect at least 20 character supported.

[WenJJ2000]

Hi @randallwhitman @stolstov ,the bugs should all have been fixed!