Replaces unecessary header includes with fwd decls

Every header is self-contained and includes only what it must to
function, relying on users include what they need in source files,
adopting a pay-what-you-use model (in particular for internal
dependencies).

opm/parser/eclipse/Units/UnitSystem.hpp

@@ -24,10 +24,10 @@
 #include <map>
 #include <memory>
 
-#include <opm/parser/eclipse/Units/Dimension.hpp>
-
 namespace Opm {
 
+    class Dimension;
+
     class UnitSystem {
     public:
         enum UnitType {