Fix #19 Introduce `CRSToCRS` class

[x4d3]

In newer versions of proj, you need to create a crs_to_crs projection and then transform coordinates with that, whereas in older versions you would pass 2 separate CRS's into the transform function.

This PR:

• Create a new class RGeo::CoordSys::CRSToCRS that is instantiated from 2 RGeo::CoordSys::Proj4 objects and stores the crs_to_crs.
• Create a singleton class RGeo::CoordSys::CRSStore that maps some combination of Proj4 objects to a CRSToCRS object. When running transform_coords, check the CRSStoreto see if the mapping exists, if not create it and store it, else get it.
• Change the underlying C function to accept the crs_to_crs instead of 2 projections and just transform the coordinates

Fix #19

[x4d3]

@keithdoggett I've moved crs_to_crs to its own file, but of course feel free to reorganise as you see fit :)

[x4d3]

@keithdoggett @BuonOmo I've applied your comment and rebased the PR. Please have another review.

[keithdoggett]

Thanks for the work @x4d3! I'll get a release out with this soon.

[keithdoggett]

This has been released as part of 3.1.0