Reformat API with encapsulated GeoDataset and GeoLayer objects

[kb173]

Instead of long function calls with paths and layer names, we'd like to switch to a more object-oriented solution: Getting a GeoDataset object from Geodot, which offers functions to get its GeoLayers (e.g. all as an Array or individual ones by name). All functionality like getting points near a position, or getting all features, will then be in this GeoLayer class. We may have to strictly differentiate between raster layers and vector layers since those offer very different functionality (-> GeoRasterLayer and GeoVectorLayer).

This has several advantages: Errors are easier to convey precisely (with is_valid() checks in each object), function calls are less convoluted, and the user has a much finer control of caching since it is in their control when a GeoDataset or GeoLayer object is deleted.

This will, of course, break compatibility with previous versions.

[kb173]

In preparation for this restructure, some classes were reworked a bit in 1db101e to remove duplicate functionality and make all the files a bit clearer and easier to look through.

[kb173]

Done and tested! The only thing left to implement has been noted in #29.