Add test for PPM save

[chrislawlor]

This PR shows coveralls reporting of two changes:

• Added a test to increase existing coverage of pytracer/image.py
• Added some new, untested code.

Setup notes:

GH action setup guide here: https://github.com/marketplace/actions/coveralls-github-action. It's really simple, just this:

      - name: coveralls
        uses: coverallsapp/github-action@v2

Have to add COVERALLS_REPO_TOKEN as a repository secret. It seems each repo gets its own token.

Have to enable comments in the repo settings:

I also had to add the @coveralls user, to allow it to add comments.

PR comments didn't seem to work right away, and they take about a min or so to show up. Subsequent commits overwrite the previous comment, which is nice.

Other notes

This repo didn't have any CI set up, so I borrowed most of the GH action setup from https://jacobian.org/til/github-actions-poetry/.

[coveralls]

Pull Request Test Coverage Report for Build 8082108182

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 95.822%

---

Totals
Change from base Build 8070512604:	0.4%
Covered Lines:	688
Relevant Lines:	718

---

💛 - Coveralls

[chrislawlor]

Since I think no one will be able to click into Coveralls, here's what I get when I click on pytracer/canvas.py link above:

[chrislawlor]

And the big Build 8070578468 link:

[chrislawlor]

If I click on the "Changed" tab at the bottom, I get a summary of coverage changes:

which clearly show that coverage of canvas.py has gone down (new untested code), and image.py has gone up (new test).