EclipseWriter: support reordering of compressed cells.

This commit removes the implicit assumption that the cells are ordered following the Eclipse Cartesian logic 
( e.g. as made sure by this two assertions:
assert(compressedToCartesianCellIdx[i] >= i);
assert(i == 0 || compressedToCartesianCellIdx[i - 1] < compressedToCartesianCellIdx[i]);
)
Now arbitrary cell orders are possible and data is remapped to the Eclipse order upon write. 
I do not understand the comment, I do not see any UnstructuredGrid object here.