In ScottPlot.Tools.XYToIntensities

New Contributors:
please review CONTRIBUTING.md

Purpose:
Improved performance for ScottPlot.Tools.XYToIntensities when using gaussian intensity mode.

The gaussian blur is separable, instead of using an n x n matrix as the kernel one can use a 1 x n matrix followed by an n x 1 matrix. This gives O(n) complexity instead of O(n^2).

Note that the previous code did not explicitly use a blur filter, but instead applied the gaussian after adding each coordinate. This is still O(n^2).

This code also precomputes the kernel, which ought to bring a performance boost of its own.

New Functionality:
N/A