Added abstract class BlackoilPropsAdInterface.

Also made BlackoilPropsAd (the class wrapping opm-core BlackoilPropertiesInterface)
inherit from it.
Make ImpesTPFAAD use abstract fluid interface instead of template.

Follow-up changes to test program. Also while there, input correct
number of perforations to add_well() calls.
Fix error in testcase specifying well prod. factors.

Also switch back to surface volume controls.