Asserts for Q_flow in MixingVolume when m_flow=0

[Mathadon]

We should add an assert that verifies if energy is conserved for zero flow if the mixing volume is configured to be steady-state energy balance.
This may detect certain singularities instead of throwing an error in the algebraic loop solver, which may result in the problems discussed in #445 .

The assert should check if the heat flow rate is such that the corresponding temperature increase stays below a certain bound.