Discuss solving method

[MichielTukker]

We (@vanmeerkerk & @MichielTukker) think we see an issue in the solve method.

1. The solver production_asset by default prescribes mass_flow. However, when it switches to prescribing Pressure, it doesn't set any equations for it's massflow (connecting to the nodes).
3. The solver node registers both continuity and a massflow=0 equation. isn't this redundant?

[samvanderzwan]

Ad 1, the production asset either sets both of its pressures or both of it mass flow rates. the mass flow rate should then follow from the system
Ad 2. Nope, the continuity equation is to couple the mass flow rate of the connecting assets to each other. The mass flow rate is zero is for its own mass flow rate.

[vanmeerkerk]

Linked to issue #166 and #167