pygr: CoordConverter: Store point in NDC coordinates (internal).

Change-Id: Id5bf90c9cef5ab1544159fecc4cf68b71346523c
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/54
Reviewed-by: Christian Felder <c.felder@fz-juelich.de>
pygr: Refactor CoordConverter / reduced functionality (API change)

In order to allow conversions between normalized device coordinates (NDC)
and world coordinates (WC) independet of width and height attributes remove
device coordinates (DC) conversion from CoordConverter and the
corresponding width and height attributes. This functionality can still be
used with the new derived class DeviceCoordConverter. Unfortunately the API
for CoordConverter will be changed by this patch but it is conventional to
call the base class CoordConverter and the derived class
DeviceCoordConverter and not vice versa.

For existing programs just replace CoordConverter with DeviceCoordConverter.

Change-Id: I94f5a3b2b5defce7be9a043df639b7aa38258618
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/55
Reviewed-by: Christian Felder <c.felder@fz-juelich.de>
pygr: New Text object for drawing text on axes or in ndc.

If a PlotAxes object has been passed to the Text object the
text will be drawn in world coordinates x, y on the axes.
If the bounding box of the text is out of bounds of corresponding
viewport the text will not be drawn by default. This can be changed
by setting hideviewport to False.

Change-Id: I1766500657177dd627093ce239ff286972f25689
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/56
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>