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/EclipseState/IOConfig/IOConfig.cpp

@@ -23,6 +23,9 @@
 
 #include <boost/lexical_cast.hpp>
 
+#include <opm/parser/eclipse/Deck/DeckItem.hpp>
+#include <opm/parser/eclipse/Deck/DeckKeyword.hpp>
+#include <opm/parser/eclipse/Deck/DeckRecord.hpp>
 #include <opm/parser/eclipse/Deck/Section.hpp>
 #include <opm/parser/eclipse/EclipseState/IOConfig/IOConfig.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/DynamicState.hpp>