EclipseWriter: use opm-parser's EclipseGrid class

this unifies the grid creation code paths with the one used by the
simulators and simplfies things.
EclipseWriter: stop making UnstructuredGrid think it is special

instead, the same interface as for Dune::CpGrid is now
used. (i.e. pass all relevant information as separate arguments.)
EclipseWriter: introduce namespace EclipseWriterDetails

it is for -- well -- the implementation details of EclipseWriter...
remove the "Eclipse" prefix from all classes in namespace "EclipseWriterDetails"

this have become become superfluous by the namespace...
EclipseWriter: Remove the non-ERT codepaths

ERT has been a requirement for opm-parser for a while now and
opm-parser is a prerequisite of opm-core, so ERT is always
available. Thus remove the stub code to avoid bitrot...
EclipseWriter: avoid "using namespace Foo" in compile-unit scope
EclipseWriter: rename some variables and functions to make the naming scheme consistent
EclipseWriter: get rid of the EclipseHandle monster

instead, the wrapper classes now call ERT directly, so it's easy to
see what a given class does. interestingly the amount of additional
code required is neglectible (or even negative).
EclipseWriter: remove some unused functions and refactor the code for Cartesian sizes
EclipseWriter: Store the Cartesian sizes internally

because it this array can be deleted after calling the
constructor. TODO (?): Do the same for the compressed-to-Cartesian
cell index map.
EclipseWriter: make it not crash if no wells are present
EclipseWriter: some minor white space changes
EclipseWriter: add unit test