Use EclipseWriter from opm-core

Note: The interface here requires opm/opm-core#424.
Whether the fluid is saturated or not is explicitly passed to the pvts

The criteria for whether the fluid is saturated or not is moved from the
within the pvt calculations to the solver, and passed to the pvt
calculations as a array of boolean values.
Merge pull request #52 from andlaus/write_binary_ecl

write out the reservoir state to the eclipse binary format
Merge branch 'mrst' of github.com:totto82/opm-autodiff into match-reference/properties
Switch condition interface to phase presence facility

Commit 4aa0eaf introduced density and viscosity evaluators into the
BlackoilPropsAdInterface 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 opm-core's commit a033329.

Update callers accordingly.
Merge pull request #69 from bska/match-reference/properties

Switch PVT Evaluators to Use Explicit External Condition Interface
make the phase-presence docstring understandable for mere mortals
Merge pull request #70 from andlaus/improve_doc

make the phase-presence docstring understandable for mere mortals