Commit a5a4d7b introduced density and viscosity evaluators into the
SinglePvtInterface that accepted an externally assignable condition
to distinguish saturated from unsaturated cases. As a result of a
few low-level technical problems with that approach, this commit
changes those affected interfaces to use the black-oil specific
'PhasePresence' facility of commit a033329 instead.
Update tests and callers accordingly.
Suggested by: @andlaus
Approved by: @atgeirr