Precise life time calculation

[robinhasse]

The minimum shares of buildings to be demolished or renovated are calculated based on life time distributions for the building, the heating system and the building shell. Currently, the time of construction or installation is assumed to be the centre of the respective time step or vintage period. While this simplification seems to work well for short time steps, the more precise solution would be to assume a uniform distribution of construction/installation across the time span and integrate. For longer time steps, this might lead to more significantly different results than we can see right now.