Also rename fastscape_basic_model to fastscape_base_model

Rename fastscape to xarray-topo.
Remove unused functions and imports in algos.
Use xrsim
Rename stream_power_model to fastscape_basic_model
Fix typo

- Add notebook: compare simpe implementation vs. framework
- Enhance numba function for stream power erosion (use scalars instead
  of arrays)
- fix wrong handling of different grid sizes in x and y

Also add docstrings for all Process run methods.
Also fix a FutureWarning with None comparison

major reorganization