Test idea: State verfication test for LYZ-Flow

[NGoetz]

LYZ-Flow has the peculiar property that the integrated flow is needed for the calculation of the differential flow. In case the integrated flow has already been computed, we reuse that result. We probably should implement a state verification test for this, to check that a) the state of the LYZ-Flow object changes through calculating integrated flow b) this has the effect of not calling the integrated flow c) intermediate results are actually saved.