Add CHELSA-W5E5 elevation data.

[juseg]

The new 'cw5e5' (CHELSA-W5E5) atmosphere datasets still contain old 'chelsa' (CHELSA-v2.1) topography. The DEM used with W5E5 is available as a single file on the ISIMIP repository and that should be used as a new elevation data source instead. It can be exposed to both hyoga.open.atmosphere() and hyoga.open.bootstrap().

[juseg]

The current choice of elevation data (CHELSA-v2.1) in hyoga.open.atmosphere() needs to be clarified in the API docs.