Add a test in an Arpack driver and a comment in documentation. Minor documentation fix
The IncNS stability solver allows for both the real and imaginary shift to be applied. The imaginary shift is realized while assembling the system matrix in the CoupledLinearNS but only if the Homogenous expansion with a SingelMod is used. At the post processing of the results from the eigen solver driver the shift is always used, leading to situations where it is not used in calculations but accounted for in post processing.
This fix adds a check at the Driver initialization level to check if imaginary Shift is used only with Homogenous and SingleMode on.