Get rid of warning when using VTK externally through Python.

When importing VTK through Python without running vtkpyython,
Python will already be initialized. This gives a warning for
that but it should be taken out. This commit gets rid of that
undesired warning.

Change-Id: Id1aaf661de25f0c688a34a3c6d747f30283deb24
Skip rendering of invalid points in PlotLine/PlotPoints

This adds code to skip the rendering of bad points in vtkPlotLine
and vtkPlotPoints. Also allows adding additional bad points with
a mask array.

This is used by the plot-over-line filter in ParaView to skip
rendering of points which are not a part of the data set.

Change-Id: I70b5caadeb21a8080b34b6ca9b24dfcd4b0f19f6
Merge branch 'skip-invalid-points-rel' into release

Change-Id: I249cb21d79e679321b73a5a6f5bceb879be30a64