Add pressure drop constraints in size optimisation

[wouterpeere]

Currently, the method to calculate the required borefield size and depth takes into account all sort of geometrical parameters, but not any pressure drop related aspects, although this is also an important aspect in the decision which option to pick.

Therefore, in this aim, this method is extended to also take into account a maximum pressure drop across the borehole and/or horizontal connections. For this last one, an assumption has be made on the longest path from one borehole to the collector.

This will be implemented after #408