Make processing dask-friendly to allow larger-than-memory and parallel processing

[alexamici]

Almost all operations can be chunked via the DEM chunks (not the image ones).

This includes:

• allow controlling the DEM chunks
• use dask-friendly xarray-sentinel helpers
• make internal functions (that need it) dask-friendly via xarray.map_blocks
• add borders to the gamma flattening processing to avoid "seeing" the borders of the blocks

The main source of dask-unfriendliness is the wide spread use of DataArray.interp with the target values on the DEM coordinates. All operations (except the gamma flattening) can be performed on ("x", "y") blocks exactly.