EclipseWriter: refactorings

here are my promised refactorings. this PR includes @rolk's changes of #554 and has been tested to work with SPE1 (it produces INIT, EGRID, UNRST and UNSMRY files which do not seem to be too unreasonable in ResInsight and the summary.x ERT tool)...

except for the patch which gets rid of the internal `EclipseHandle` class, everything should be small and self-contained...