Initial steps towards a dynamic exhalation rate

This MR introduces the possibility to define different activity types, both for the infected and exposed populations. Initially, only the concentration is possible to be calculated.

Next step is to include the calculation of the dose when the activity is dynamic, i.e. dependent on time.

Script file:CO2_fit.py

This is the first step towards the possibility of having the CO2 fitting algorithm calculating different breathing rates for different activity transition times.