Make io subdir

This moves some classes and utilities to the opm/core/io subdir. The most disruptive change is that the include path of the EclipseGridParser class changes. All needed changes have been PRd in opm-polymer, dune-cornerpoint, opm-porsol and opm-upscaling.

If there are any other repos that needs changing, please say so and I'll hold off merging this.