Pierre/class vs classref typecasts

This is an entirely different approach to try to solve the explicit typecast from classref to class or class to classref.

The first two commits remove some direct tpyecast, simply by inserting an intermediate typecast to Pointer.

The third commit generates an error when explicit typecast from classref to class or from class to classref is done, as this is serious source of potential errors.

This Merge Request superseeds !182 (closed)

Pierre