Custom variables when running aeroval

[dulte]

Add possibility to add custom variables without hardcoding

Hilde and others have asked for the possibility for users to add custom variables. As of now this can only be done by hardcoding the data. I want it to be possible via aeroval configs.

Several steps are needed

• [x] Add option for custom EMEP mapping (species name to EMEP name)
• [x] Fix model_read_aux. As of now, it fails serialization due to having a function in the dict
• [ ] Add support for aux vars in pyaro reading
• [x] Add option for extending ´variables.ini´ runtime
• [ ] Document all features!!!
• [ ] Add tutorial for this during the EMEP workshop

Is being addressed in #1147