[docs] Explain assumptions in simulation

[jnnr]

Make sure that these assumptions are mentioned in the documentation of the simulation models (these are review suggestions from #42).

• properties of water are set globally, that means the idea is to take kind of average values, e.g. if you consider a dhs system with 80°C forward and 50°C return, the properties at 65 °C could be a good choice?
• The hydraulic and thermal equations are solved independently. This is possible due to the constant properties
• The flow direction must be known before simulation. Maybe that is already clear, since here tree networks are considered.
• At the moment, there is only the option of one producer.