Add isPinchActive() and getPinchThresholdThickness() members.

Now reading the optional PINCH keyword from the GRID section.
Add test for pinch features of EclipseGrid.
Make getPinchThresholdThickness() throw if isPinchActive() is false.
More tests for proper pinch-related behaviour.

Now also check behaviour for no-pinch case.
Follow opm-parser naming conventions for data members.
Name magic constant (invalidThickness).
Merge pull request #282 from atgeirr/pinch-eclipsegrid

Add pinch features to EclipseGrid class