[hiopy] fractional masks - test and validate

Coyote has support for fractional masks when it is used as a source. This should be used to solve the coarsen problem when cells actually covers less meaning full data. (see https://gitlab.gwdg.de/ican/hiopy/-/issues/15 and https://gitlab.gwdg.de/ican/hiopy/-/issues/10)

hiopy already has support for it but it lacks documentation, testing, an example, validation and utilities to set the dataset up (configure.py)

Add an example and reference it in the documentation
Test the example in the CI
validate the produced data in a real world setup
extend configure.py for setting the fractional mask variable up. (Interpolation from somewhere?!, discussion needed)