Earthkit-hydro

Make compatible for inclusion in pip install earthkit.

Ensure compatibility with pypi.

First release with the following features:
- Support for PCRaster, CaMa-Flood and HydroSHEDS river networks
- Computing upstream statistics and statistics over catchments and subcatchments
- Finding catchments and subcatchments
- Calculation of upstream or downstream fields
- Handle arbitrary missing values
- Handle N-dimensional fields