develop density plots

[o-smirnov]

When making plots from millions of visibility points (if averaging is not in use, for example), we might want to consider density plots instead.

For example, instead of making a plot of 10 million individual points on an UVdistance-Amplitude axis, you could make a UVdistance-Amplitude image, where each pixel is a counter of how many points fell into that bin. Essentially a 2D historgram.

[o-smirnov]

Probably using #19?