CO₂ Concentration Plot

Changes to include CO₂ concentration plot in the report. Added the possibility to export the CO₂ data as CSV.

The population for the CO2ConcentrationModel is dynamic, built from the merge of number and presence attributes of the infected and exposed populations.

The previous method to generate the alternative scenarios plot was updated to be generic, so that we could use it to also generate the CO₂ plot.

Implemented - example:

infected = InfectedPopulation(number=10, presence=SpecificInterval(present_times=((8, 12), (13, 17))), ...

exposed = Population(number=20, presence=SpecificInterval(present_times=((9, 11), (14, 16))), ...

co2_emitters = Population(number=IntPiecewiseConstant(transition_times=(8, 9, 11, 12, 13, 14, 16, 17), values=(10, 30, 10, 0, 10, 30, 10)), ...

Result:

closes #297 (closed) and #307 (closed)