Zero removal rate case (mainly for CO2 concentration with no ventilation)

• Implement the right equation when RR=0 in _ConcentrationmodelBase._normed_concentration: instead of

        # If RR is 0, conc_limit does not play a role but its computation 
        # would raise an error -> we set it to zero.
        try:
            conc_limit = self._normed_concentration_limit(next_state_change_time)
        except ZeroDivisionError:
            conc_limit = 0.

        t_last_state_change = self.last_state_change(time)
        conc_at_last_state_change = self._normed_concentration_cached(t_last_state_change)

        delta_time = time - t_last_state_change
        fac = np.exp(-RR * delta_time)
        return conc_limit * (1 - fac) + conc_at_last_state_change * fac

we should put something like:

        conc_at_last_state_change = self._normed_concentration_cached(t_last_state_change)
        t_last_state_change = self.last_state_change(time)
        delta_time = time - t_last_state_change
        if RR != 0.:
            conc_limit = self._normed_concentration_limit(next_state_change_time)
            fac = np.exp(-RR * delta_time)
            return conc_limit * (1 - fac) + conc_at_last_state_change * fac
        else:
            # If RR is 0, the concentration is linear with time.
            return delta_time/self.room.volume + conc_at_last_state_change

(and create some corresponding tests - checking in particular that the value of the CO2 concentration when decreasing the ventilation rate to a very small value, gets close to the value at exactly zero).

• Then, we can remove the minimum 1e-6 ACH residual ventilation value in CO2ConcentrationModel.