DDT: add a graph showing the volume (person.day)

Purpose: evaluate the usage/volume of each travel mode in the lab Usage is measured in person.day.

To do so, we count 1 person.day for a given travel mode each day it's involved in a travel of a poll's respondent. This is then corrected by the number of weeks in the current year and extrapolated to all persons in the lab (as it's done for the distances and emissions)

Fix #125 (closed)

Une idée (un labo avec 2 profs/chercheurs, 1 ingé, 1 étudiant, 1 trajet voiture par semaine)