Threshold pressure

This adds the THPRES feature to the black oil simulators. The feature has been through limited testing:
 - It still runs SPE9 with identical results in the same time (this is a case with no THPRES).
 - With artificial THPRES equal to zero applied to all faces, the results are still identical (but maybe with a tiny time penalty).
 - With artificial THPRES that is nonzero, the simulator runs, but I have not compared the results to any known good ones.

The code uses the thresholdPressures() function from opm-core, so OPM/opm-core#639 and OPM/opm-parser#292 are required.
I was surprised that I needed it, for the same reason you give. However we are in a template context, and that decides the issue. If I am not mistaken, earlier gcc versions accepted dropping the template in this context, but it is not correct C++. It should be, though!