Adding Incompressible Fluids EOS to FPROPS

A wide range of research and engineering applications involve computing the thermodynamic properties of substances that are in the incompressible regime. I propose to append a model for incompressible fluids to the existing models in FPROPS. The simplifications in the formulation of incompressible EOS can significantly increase the performance of the computations compared to those with the general Helmholtz based EOS.