[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).