Re-organize raster and vector functionalities as separate functions

[rhugonnet]

Already mentioned when discussing #383, we need to move out all function of Raster and Vector to live outside the class, both to facilitate the Xarray accessor and tests.

Here's an idea of structure, adding new modules:

In geoutils/:

raster/ -- accessor.py (Xarray raster accessor) -- distance.py (proximity) -- georef.py (for reproject, crop)

vector/ -- geometric.py (buffer_with_overlap) -- georef.py (reproject, crop)

raster_vector_interface.py (rasterize, polygonize, create_mask, to_pointcloud)