Create 2-d heatmap of wealth histogram over time

[divilian]

We realized we needed this in issue #8 discussion, but it got lost.

To be clear, the action item is this:

• Currently, if :make_anims is true, the SPECnet code generates a histogram of agent wealths for each iteration, and then puts them together in an animated .gif, where each frame is one iteration of the simulation. (See plot_iteration_graphs() at end of sim.jl.)
• In addition to this, we want a non-animated graphic that gives the same information. The solution is to have the x-axis represent wealth bin, the y-axis represent iteration number, and the "z-axis" (color) represent the number of agents in that iteration who have wealth in that region (or, put another way, the height of the histogram bar in that bin). The simulation should output a wealth2d.png file in the tempdir() directory with this heatmap.

[divilian]

We didn't actually mention this in yesterday's Zoom call as one of the things we wanted for phase 1. To be clear, this is a heatmap of a single sim that shows time on its y-axis, rather than a heatmap of an entire param sweep that shows the swept parameter on its y-axis. I'm putting this in phase 2 for now. Squawk if you disagree, @venkatachalapathy or @will-nehrboss.