Document usage of visualize_pointclouds.ipynb and visualize_reconstructions.ipynb

[pgarrison]

These two notebooks are required for producing panels c and d of figures 2-4 (one script) and 5-6 (the other). The documentation should give the exact steps required to produce the figures.

[pgarrison]

This will require documenting how to download the raw images for pcna and cytoplasmic punctate structures (endosomes, peroxisomes, and centrioles), since those are inputs for this step.