qtgr, pygr: Zoom at origin p0.

qtgr: Zoom at mouse location p0
pygr: Zoom at origin p0 (which corresponds to
      the mouse location if called from qtgr).
pygr: Implement zoom in NDC space.

Calculations in NDC space are regardless of e.g. log scaling.
pygr: CoordConverter: replace gr.wctondc call.

- replaced 'gr.wctondc' call in CoordConverter setWC method with formula.
- Interface change: setWC method requires a viewport attribute to
                    calculate normalized coordinates.

Conflicts:
	lib/gr/python/gr/pygr/__init__.py
pygr: Implement panning in NDC space.

Calculations in NDC space are regardless of e.g. log scaling.