Add partitioned raster data structure

[kordejong]

A raster is needed in case of operations that need to know about the cell size, bounding box, coordinate reference system, ... Examples are: slope, resample, warp, ...

One option is to re-use the partitioned array and aggregate it with crs info.

Afterwards, existing algorithms need to be overloaded for rasters. Algorithms that don't need CRS info can simply forward to the overload accepting arrays. I/O routines should be updated to use raster instances.