Reduce number of attributes currently used by the development version

[nikosbosse]

The current development version of scoringutils makes use of attributes to keep track of relevant information. However, attributes may be brittle and can be lost along the way if the user does any other operation on the data between function calls, so we shouldn't rely on them if we can avoid that.

We can avoid attributes forecast_unit and forecast_type by referring to the functions get_forecast_unit() and get_forecast_type() and make active use of them in the print method.

We can avoid attributes messages and warnings by simply throwing messages and warnings and letting the user keep track of them.

Those attributes should be removed from the current code.

[nikosbosse]

We simplify everything for now and if users feel that they need an extra check for e.g. changes in the forecast unit, we can add attributes back in later on