Cleans up headers to improve build preformance

This is an effort to improve build performance.  Several includes
scattered across the project are either unused or partially used (i.e.
just used to import a type name, not depending on the actual contents of
the header file).

Replaces a lot of these includes with forward declarations.

opm/parser/eclipse/EclipseState/IOConfig/tests/IOConfigTest.cpp

@@ -26,7 +26,10 @@
 #include <opm/parser/eclipse/Parser/Parser.hpp>
 #include <opm/parser/eclipse/Parser/ParseMode.hpp>
 #include <opm/parser/eclipse/EclipseState/EclipseState.hpp>
+#include <opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp>
 #include <opm/parser/eclipse/EclipseState/IOConfig/IOConfig.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Schedule.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/TimeMap.hpp>
 
 
 using namespace Opm;