Build system: mark ERT as REQUIRED

de facto, this does not change anything because opm-parser is required
and it has a hard dependency on ERT. It is good style to declare all
prerequisites explicitly in each module, though. Thanks to [at]bska
for the catch.
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