Add new ContinuousScatterPlot filter

This new filtered designed for bi-variate analysis builds the continuous scatterplot of a 3D tetrahedralized mesh for two given scalar point fields.

The continuous scatterplot is an extension of the discrete scatterplot for continuous bi-variate analysis. The constructed DataSet consists of triangle-shaped cells, whose coordinates on the 2D plane represent respectively the values of both scalar fields. Triangles' points are associated with a scalar field, representing the density of values in the data domain.

This VTK-m implementation is based on the algorithm presented in the paper "Continuous Scatterplots" by S. Bachthaler and D. Weiskopf. I used the TTK implementation as a reference.

Applying this filter to TTK examples' 'mechanical.vtu' and comparing it to the reference implementation gives the following (rendered here in Paraview)

TTK Render

VTK-m render

The main difference between both filters is that TTK re-discretizes the output into a 2D Image Data using raycasting after computing the triangles, whereas in VTK-m we have the bare triangles as an output.

This means, to get the density on a specific point using VTK-m output, you would need to raycast to sum all contributions of interpolated triangles passing through this point. Future work could include 2D Delaunay triangulation to address this shortcoming.