Concise definition of branch properties

[dlohmeier]

In some calculations (derivatives) we need certain fluid properties (i.e. state values) that could change with the propagation along a branch, but must be used to derive other values (e.g. pressure). Of course this is strongly related to the finite volume method we use and often a question of good discretization, but still we need a way to define such a state value for the whole finite volume of the considered branch. So, we should have a look at how to best describe the properties density, viscosity and heat capacity (others?) for the whole branch in order to not make our calculations a bad approach in case of "not ideal" discretization. The current implementation can be found in the properties toolbox.