Fixed bug on frequency and duration on natural ventilation

Fixed https://gitlab.cern.ch/cara/cara/-/issues/261

The calculation of the start time for the PeriodicInterval was considering the minutes_since_midnight and not the hours since midnight. This was leading to wrong results when calculating the respective interval boundings.

Fixed by dividing the start time by 60 - minutes to hours.

http://localhost:8080/calculator?infected_people=1&location_name=Geneva%2C+CHE&location_latitude=46.20833&location_longitude=6.14275&opening_distance=0.5&room_number=2&room_volume=250.0&simulation_name=Test&total_people=2&ventilation_type=natural_ventilation&virus_type=SARS_CoV_2_OMICRON&volume_type=room_volume_explicit&windows_duration=10.0&windows_frequency=60.0&window_height=1.0&window_type=window_hinged&window_width=1.0&windows_number=1&window_opening_regime=windows_open_periodically