fix: pygr: Set markercolor if just polymarkers drawn in PlotCurve.

Change-Id: Ia333cf485dd4aebf3616d9329aeec9be47f70dfc
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/121
Reviewed-by: Christian Felder <c.felder@fz-juelich.de>
fix: pygr: GKS: Number of points is invalid in routine POLYLINE.

Do not draw polyline in PlotCurve if #points < 2.

Change-Id: I592b02d64885217cfa937a6ecbd8d9e4eef22080
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/120
Reviewed-by: Christian Felder <c.felder@fz-juelich.de>
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>