moving drag curve per voltage level to single drag curve for EM. updating GUI, input data provider, data apapter, etc.

- engine form: use correct filebrowser instance
- vtp job: plot second fuel if available
- json writer set engine-only mode correctly
- test-project: do not use classes from GUI but already available classes from test project

Added checks if some Required Fields are filled (Electric Motor File, EM ADC Loss Map). Otherwise application would crash.

saving and loading battery system works, write modal results for batterysystem (total/average of all batteries / battery strings)

update electric motor form to use two voltage levels, adapt input data, json reader, json writer for electric motor, adapt testcases to make it compile