dependence on corsika-data
since the qgsjetII tables are in the corsika-data repository and the qgsjet tests are configured to run by default, building corsika according to README.md currently fails. cmake only prints a warning that the variable CORSIKA_DATA has to be set..
maybe we should make it an error and always require corsika-data to be installed. maybe we can set cmake up to get it? I do not think we want qgsjet to be optional.