core: add SWIG data object

CMakeLists.txt

@@ -128,6 +128,7 @@ SET(${PROJECT_NAME}_HEADERS
   include/eigenpy/ufunc.hpp
   include/eigenpy/register.hpp
   include/eigenpy/stride.hpp
+  include/eigenpy/swig.hpp
   include/eigenpy/version.hpp
 )
 

include/eigenpy/swig.hpp

+//
+// Copyright (c) 2020 INRIA
+//
+
+#ifndef __eigenpy_swig_hpp__
+#define __eigenpy_swig_hpp__
+
+namespace eigenpy
+{
+  struct PySwigObject
+  {
+    PyObject_HEAD
+    void * ptr;
+    const char * desc;
+  };
+}
+
+#endif // ifndef __eigenpy_swig_hpp__