Unifies the phase indices with the ones in opm-core.

The initial definition of the phase indices seems to be in
opm/core/props/BlackoilPhases.hpp. Nevertheless there were
several redefinitions of the same or similar enums (either
Aqua, Liquid, and Vapor, or Water, Oil, and Gas). Surprisingly
most often these definitions did not use the original values.
This is bound to break if there is a change upstream.

This patch limits the definition to one place in opm-autodiff,
namely opm/autodiff/BlackoilPropsAdInterface.hpp. To avoid
downstream confusion we define both the Water and Aqua triplets.
In addition we define the maximum number of phases to use at compile
time.

opm/autodiff/BlackoilPropsAdInterface.hpp

@@ -78,7 +78,12 @@ namespace Opm
         // ------ Canonical named indices for each phase ------
 
         /// Canonical named indices for each phase.
-        enum PhaseIndex { Water = 0, Oil = 1, Gas = 2 };
+        enum PhaseIndex { Water = BlackoilPhases::Aqua, Oil = BlackoilPhases::Liquid,
+                          Gas = BlackoilPhases::Vapour,
+                          Aqua = BlackoilPhases::Aqua,
+                          Liquid = BlackoilPhases::Liquid,
+                          Vapour = BlackoilPhases::Vapour,
+                          MaxNumPhases = BlackoilPhases::MaxNumPhases};
 
         // ------ Density ------