Implement Mixer/Splitter unit operation

[schmoelder]

In CADET-Process, flow rates are only specified for Sources and CSTRs. The value of the flow rate for other units is automatically calculated using output state variables that specify the distribution of flow to units following downstream (basically acting as a valve).

For the setup of more complex systems of unit operations, especially those with internal recycles (e.g. SMB), a unit without an actual model would be helpful which just mixes the inputs and distributes the output streams. This could be achieved by setting a CSTR's volume to zero, however, this currently raises a timeout error.

If this is not consistent with the design philosophy of CSTRs, it would also be possible to create a new unit operation (e.g. a Mixer/Splitter/Valve unit).

[sleweke]

The CSTR has problems with vanishing volume. It should be able to work with V=0 for some singular time points, but not for whole time intervals.

However, I see your point and there's nothing against it. We can simply add a unit operation for this purpose. It can be based on the Outlet unit operation, which only needs an actual outlet.

We should choose the name carefully. though. A physical valve has dead volume, which may be better modeled by a CSTR. Mixer or Splitter seem better suited.