fix a few CLang warnings

most of them quite insignificant, but still annoying. The only
exception is the warning about the changed alignment for the 'work'
argument of spu_implicit_assemble(). AFAICT, the only reason why it
worked was that the pointer produced by malloc() was passed
directly. (malloc() seems to fulfill all alignment criteria.) To fix
this, I've changed that argument's type from char* to double*.

opm/core/props/BlackoilPropertiesInterface.hpp

@@ -23,7 +23,7 @@
 namespace Opm
 {
 
-    class PhaseUsage;
+    struct PhaseUsage;
 
     /// Abstract base class for blackoil fluid and reservoir properties.
     /// Supports variable number of spatial dimensions, called D.