rework relative transmission risk

[charlie0614]

The relative transmission risk is dependent on the occupancy of the location. For this dependency we use a function f but it is not clear what f should look like. In the current implementation we have f=num_persons/capacity_volume which leads to E'=k * num_persons/capacity_volume * E, i.e. we don't use the capacity in persons of the location. We should think about the definition of f, i.e. is it linear or something else?

This is currently implemented in the file location.cpp function compute_relative_transmission_risk.