Add Checks around Proj4 and CRSToCRS objects

[keithdoggett]

Fixes #25

• Add crs? method to Proj4 objects. This checks if the underlying pj struct is a CRS type (https://proj.org/development/reference/datatypes.html?highlight=pj_type#c.PJ_TYPE).
• Raise RGeo::Error::InvalidProjection if PROJ cannot parse the input string when creating a projection. Currently, it returns nil, but it's possible that further operations break because of that and cause a segfault.
  • The reason for this is that it doesn't return nil, but returns a Proj4 object where the underlying RGeo_Proj4Data struct is nil, so you can still call methods on it.
• Raise InvalidProjection when a non-crs projection is attempting to create a CRSToCRS object.
• Re-organize C code a bit.

[keithdoggett]

@BuonOmo thanks for the notes. And yeah I wanted to discuss if this was considered breaking. I think you're correct that it is, so I agree it should point to a different branch (v4-dev).

If that's the case I'll open up another issue with some other ideas I had that we can throw into a new major version.

[keithdoggett]

@BuonOmo I made all of the changes mentioned except for the one regarding marking the modules which needs more investigation. I'll also open up an issue for v4 shortly.

[keithdoggett]

@x4d3 if you have some time perhaps you could take a look through these changes since they affect the CRSToCRS module, which you're more familiar with than I am, and let me know if these make sense. Thanks!

[x4d3]

🚀