Implement Spatial Join of RasterCollections

[lukasValentin]

It would beneficial if RasterCollection instances could be joined spatially (RasterCollection.join()).

This functionality would allow to

• perform spatial joins, i.e., merge raster datasets (a.k.a. mosaic generation) from adjacent tiles or different data souces
• check for spatial overlap of rasters (implementing classic GIS functionality and topological operators)

Currently, we still have to rely on rasterio. This is actually not really a problem but we have to perform more I/O operations because we have to write files to disk, merge them using rasterio and read the result back into EOdal.

The questions are:

• is it feasible to implement it in "pure" EOdal style?
• is there any advantage over using rasterio?

If none of the questions above is answered positively, I'd suggest to continue using this function and implement it on the RasterCollection class level in join.