[material] Add option to PengRobinson::computeMolarVolume
- Ignoring the first two roots of the equation for Z in the smallest one is zero or negative. There was a comment documenting that behavior, but it was not implemented. - Throwing NumericalProblem if no positive Z has been found or the number of roots is 2 or more than 3 (no check was in place) - Throwing NumericalProblem if the found molar volume turned out to be zero, negative, or NaN. (Replacing assert) - Added handleUnphysicalPhase flag (true by default). If false, the single-root case is not checked for critical state of the fluid (otherwise, Michelsen test for a mix of hydrocarbons does not work).