Improve convectiveHeatFlow

I see to possible improvements:

• This component could be made more generic representing not only convective but also resistive or combined thermal resistances.
• The k and A parameters are arrays: since this is a quite approximative/simplistic model, I cannot see when the user would introduce different values for each node. Potentially in case of calibration (not even sure...) ?

TO BE DONE → It has to be checked whether this component is already used in other components of ThermoSysPro