Add swimming pool model (incl. water handling units)

[larissakuehn]

So far the AixLib library does not contain a swimmming pool model.

With this issue we want to add a model for a swimming pool. The focus is on creating a model for the heat and water demand of a pool that is tempered to a desired level. Therefore the model does not only include the pool itself with all relevant heat and mass flows but also the connected water treatment cuircuit as shown in the following figure: