Fix fallout from opm-core refactoring

This commit updates the call to initFacePressure() to account for
recent changes in opm-core (PR OPM/opm-core#518) that modify some of
the calling interfaces.  No functional change.
Reduce nesting level of initFacePressure

Commit 4564258 updated the call to function initFacePressure() to
accommodate recent changes to the interface.  In the process we
introduced a nested scope that's really unneeded here.  Remove it.

Suggested by: @rolk.
Merge pull request #62 from bska/fallout-from-refactoring

Fix fallout from opm-core refactoring