Added the interfacial pressure-work terms according to:

Ishii, M., Hibiki, T.,
Thermo-fluid dynamics of two-phase flow,
ISBN-10: 0-387-28321-8, 2006

While this is the most common approach to handling the interfacial
pressure-work it introduces numerical stability issues in regions of low
phase-fraction and rapid flow deformation.  To alleviate this problem an
optional limiter may be applied to the pressure-work term in either of
the energy forms.  This may specified in the
"thermophysicalProperties.<phase>" file, e.g.

pressureWorkAlphaLimit 1e-3;

which sets the pressure work term to 0 for phase-fractions below 1e-3.

For particularly unstable cases a limit of 1e-2 may be necessary.