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.
> earlier gcc versions accepted dropping the template in this context

I'd be very surprised if recent would not accept this either. Also, I suppose that starting with c++-2011 this keyword is probably optional, but not required. (Just judging from the `typename` qualifier which can be used outside of template contexts in c++-2011. I did not waste my time to fully understand these subtleties of the standard here, though...)