Add PointCloud spatial distribution

[gonuke]

Description

Allows users to define a list of points in space, each with a different relative intensity, to be sampled discretely. This is a valid SpatialDistribution and can be used anywhere that a SpatialDistribution is valid.

Fixes #3159

Checklist

• [x] I have performed a self-review of my own code
• [x] I have run clang-format (version 15) on any C++ source files (if applicable)
• [x] I have followed the style guidelines for Python source files (if applicable)
• [x] I have made corresponding changes to the documentation (if applicable)
• [x] I have added tests that prove my fix is effective or that my feature works (if applicable)