Modified time dependence of concentration when removal rate is none

This MR implements a change in the equation when RR=0 (_ConcentrationmodelBase._normed_concentration). With these changes there is no need to catch ZeroDivisionErrors given that conc_limit is only calculated when the removal rate (RR) is different than 0..

If the removal rate is 0., the concentration is linear with time, which means that the previously developed test (test_zero_ventilation_rate) had to be updated to consider the delta_time/self.room.volume. Other test entries were added to cover the cases when the value of the ventilation is close to 0..

Closes #319 (closed)