core: add forward declaration of main classes

include/eigenpy/fwd.hpp

 /*
- * Copyright 2014-2019, CNRS
- * Copyright 2018-2019, INRIA
+ * Copyright 2014-2020 CNRS INRIA
  */
 
 #ifndef __eigenpy_fwd_hpp__
@@ -27,4 +26,10 @@
 
 #include "eigenpy/expose.hpp"
 
+namespace eigenpy
+{
+  template<typename MatType> struct EigenToPy;
+  template<typename MatType> struct EigenFromPy;
+}
+
 #endif // ifndef __eigenpy_fwd_hpp__