Move GridProperty< T > to source file

GridProperty< T > has only two sensible types to parametrise, int and
double. Moves the implementation and instantiation of GridProperty to
GridProperty.cpp. This also applies to GridPropertyInitializers which
has also been moved to source files.

Results are a cleaner header file (for reading & understanding the
interface) and faster compilations.

opm/parser/eclipse/EclipseState/SimulationConfig/tests/ThresholdPressureTest.cpp

@@ -28,6 +28,7 @@
 #include <boost/filesystem.hpp>
 #include <opm/common/utility/platform_dependent/reenable_warnings.h>
 
+#include <opm/parser/eclipse/Deck/Deck.hpp>
 #include <opm/parser/eclipse/EclipseState/SimulationConfig/ThresholdPressure.hpp>
 #include <opm/parser/eclipse/Parser/Parser.hpp>
 #include <opm/parser/eclipse/Parser/ParseMode.hpp>