Enhanced the parallel coordinates chart for doubles

The parallel coordinates did not correctly handle double precision
input, added support to vtkPlotParallelCoordinates and created a
new test to verify it. Also some very minor clean up in the other
test, and fixed variable naming for consistency.

Change-Id: I4d688857dcfe86b044b8284789e7873f698d8820
Merge topic 'parallel-coords-double' into master

347fb125 Enhanced the parallel coordinates chart for doubles