Refactor BlackoilEclipseOutputWriter.

Before the release I'd like to see the following refactoring:
 - rename class to EclipseOutputWriter
 - generalize its use to cover more cases, including two-phase (see also #420)
 - replace hardcoded units with Units.hpp usage
 - make it exception safe (basic guarantee is enough).

Much of this is from @bska's original comment in OPM/opm-autodiff#52.