Esmvaltool optional #447
Description
ESMValTool (and ESMValCore) are only used when generating certain forcings. However forcing can also be generated with CaravanForcing which does not need ESMValTool.
It would be nice if you could install and use ewatercycle package without having ESMValTool installed.
When ESMValTool is an optional dependency we should be able to install this package without conda or mamba, which is also nice.
Spawned from discussion at #442