Unify grid construction from deck, allow PINCH

With this, the grid construction process goes through EclipseGrid every time, and PINCH is supported.

The z_tolerance argument has been removed from the construction methods, use PINCH in the deck instead. This is warned about in places where users could specify z_tolerance.

Merging this has downstream consequences, and those PRs should be considered together with this one.

Requires OPM/opm-parser#284.