Move to opm parser

this adds methods for opm-parser to the classes which were missing and converts all examples and tests to the opm-parser, i.e. the EclipseGridParser is completely unused after this PR.

This PR is a prerequisite for the conversion other modules (which I have done already but I don't want to have too many things "in-flight", so I'll wait until this PR has been merged). It does not regress any CTests on my machine, but given the fact that the CTests are not very comprehensive for opm-core, that probably does not mean too much, so please test thoroughly...