#858 RichPNG loose ends

[Jeremy Faden]

I noticed a number of loose ends with RichPNG were never tied:
https://github.com/autoplot/documentation/blob/main/md/richPng.md

Since Autoplot was the only software which ever used the feature, I should just define something to finish these off. Specifically:

1. put an optional flipped property on the axes.
2. add richPNG metadata to PDF output.
3. make a rich png code for Python.
4. add an optional page timerange property which is iso8601 range.
5. document minpixel and maxpixel of colorbar.
6. layout for size, right, left, top, bottom, minpixel, maxpixel can be strings, and can contain "em" or "in" or "cm" for units.
7. json schema documenting the format.
8. consider making units, top, bottom, minpixel, maxpixel, and type all optional.
9. finalize the documentation -- I noticed it still had upside-down axes.

[Jeremy Faden]

(9) I was mistaken about the y-axis. It is (0,0) in the upper-left corner.