Implement a method to retrieve the mass_flow and fluid branches from the strucutre matrix

[fwitte]

The structure matrix includes topological structure but also contextual structure in the sense, that specified equations are also inserted. It would be nice, if it would be possible to generate these two elements separately, because then on top of the initial model simplification we can also identify easily where exactly the same mass flows and fluid compositions are available (and not where linearly dependent mass flows and fluid compositions are available).