Purpose: Improvement for HDFFV-10004

Description:
    When adding wrappers for H5Lexists, a new class, LinkAccPropList, was
    added to the C++ API, triggered complicated circular dependencies.  Thus,
    some improvement was made to resolve the problems.
    - Replaced existing functions openXxxType with individual type constructors
      + Added individual XxxType constructors to replace the existing functions
        openXxxType because it's rather awkward to use these functions.
      + Moved openXxxType from H5Location back to CommonFG
      + Put back CommonFG as a baseclass of Group for openXxxType functions.
      + This replacement should improve usability and prevent the problem of
        circular dependencies.
    - Removed overloaded constructor that takes an Attribute when there is
        already one that takes H5Location because Attribute inherits from
        H5Location now.
Platforms tested:
    Linux/32 2.6 (jam)
    Darwin (osx1010test)
    Linux/64 (platypus)

c++/src/H5DataSet.cpp

@@ -20,6 +20,7 @@
 #endif
 #include <string>
 
+#include "H5private.h"		// for HDfree
 #include "H5Include.h"
 #include "H5Exception.h"
 #include "H5IdComponent.h"
@@ -34,11 +35,8 @@
 #include "H5DataType.h"
 #include "H5DataSpace.h"
 #include "H5AbstractDs.h"
-#include "H5Group.h"
-#include "H5File.h"
 #include "H5Attribute.h"
 #include "H5DataSet.h"
-#include "H5private.h"		// for HDfree
 
 namespace H5 {
 using std::cerr;