Disentangle infected and exposed presence times in model_generator

Correction of improper times used when generating the report of the calculator: presence time were taken from the infected only, and identical for both exposed and infected. Now infected and exposed start and finish time are different (as provided in the form) and the plot goes from the minimum start time until the maximum finish time (as suggested by @jdevine )

It would be perfect if someone could test the calculator app with this (I still did not install locally the calculator...)

Should solve #99 (closed)