Fitting results: added flow rate data (l/s/p)

This MR adds two new columns to the result of the CO2 fitting algorithm, corresponding to the ventilation flow rate in L/s and L/s/person (being the last one only added upon definition of a new input - explained below).

Before, the output of the algorithm was only the air changes per hour (ACH). To obtain the flow rate in L/s, the ACH is multiplied by:

• 1/3600 to obtain the result in air changes per second
• room volume
• 1000 to convert volume to liters (1m^3 == 1000L)

To obtain the flow rate in L/s/person the previous flow rate is divided by the "maximum occupation – design limit" of the room (new optional input : room_capacity)

Given the foreseen dynamic occupancy feature, the number (int or IntPiecewiseConstant) and presence (None or Interval) attributes of the CO2DataModel class were replaced by the following attribute: occupancy (IntPiecewiseConstant).

Added logical validation to the new optional input (room_capacity): when defined, needs to be an integer higher than 0.

Closes #416 (closed)