Unsteady incompressible solvers use constant density assumption.

[bigfooted]

Is your feature request related to a problem? Please describe.
Unsteady species transport, including combustion, uses a constant density assumption.
The density at timesteps n+1, n and n-1 (and possibly also Cp) needs to be stored for incompressible unsteady flows.
SetResidual_DualTime needs to be adjusted in CIncEulerSolver.cpp and CScalarSolver.inl

[bigfooted]

closed by #2641