Adjust plantperformance.py metrics

plantperformance needs to be calculated in every timestep.
Also, it must be adjusted to feature an arbitrary amount of bioreactor outputs.
It should contain Kjeldahl N, total N, total COD and BOD5 concentrations
Lastly, it must be extended to feature other metrics (of other reactors) as well - this might be times, where sludge temperature is below a certain threshold