should check zero m_flow_nominal in movers

[Mathadon]

I mistakenly tried to set a constant flow rate of 0 kg/s by setting m_flow_nominal in FlowControlled_m_flow instead of by setting constantMassFlowRate. This produces the following error

Model error - division by zero: (0.1*(pump1.eff.dpMax*pump1.eff.delta^2)) / (pump1.eff.V_flow_max) = (5) / (0)

It would be good to add an assert that verifies that m_flow_nominal is strictly positive?

[Mathadon]

@mwetter do you agree?

[mwetter]

A check on m_flow_nominal > eps in the fan would be good.