add example to calculate psidiag(Rdiag,Zdiag) at specific diagnostic Rdiag,Zdiag values from equilibrium psi(R,Z), as well as rhopol, rhotor etc. This is used in gdat,AUG now

git-svn-id: https://spcsvn.epfl.ch/repos/TCV/gdat/trunk@4424 d63d8f72-b253-0410-a779-e742ad2e26cf