Eclipse writer remove eclipse handle

this PR is based on #559. The only difference is the last (quite big) patch. Once #559 is merge only this patch should appear in the series...
Hmmm - this seems to be wrong; as I read the current the actnum which is written to disk is exactly the same actnum as given in the input deck? However the opm processing of the Eclipse cornerpoint input will in general have deactivated additional cells compared to the input actnum, 

The final writing must therefor be based on the actnum information from UnstructuredGrid (or the like), i.e. `compressedToCartesianCellIdx` in this case.