pimpl management good practice

[mwestphal (Mathieu Westphal (Kitware))]

@ben.boeckel

Please don’t use shared_ptr, but unique_ptr instead. There are zero reasons why these classes should escape and be cooperatively shared around. Also, just do Internals(new vtkInternals) instead of .reset().

Most definitely.

@utkarshayachit unique_ptr impl seems the most reasonnable one.