Add a facility for checking/assigning presence of free phases

This is intentionally black-oil specific because we presently do no
know how to handle other cases (e.g., more phases or number of phases
different from number of components).
Switch condition interface to phase presence facility

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