Various improvements and fixes

[odow]

I've refactored the tests with a view to eventually incorporating them into JuMP.

When writing new tests, we should keep them in this single file and add new test functions, rather than adding new lines to exiting functions.

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@e15ddf0). Click here to learn what that means. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main       #4   +/-   ##
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  Coverage        ?   78.77%           
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  Files           ?        3           
  Lines           ?      179           
  Branches        ?        0           
=======================================
  Hits            ?      141           
  Misses          ?       38           
  Partials        ?        0           

:mega: Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[odow]

Okay, I now understand the code a lot better. It's a really nice job! I think we need some codecov to see which methods are being hit, and a lot more tests. But I like the design, and I think this would be a lot of use to people writing large models for physical systems.

We can probably merge this PR with rebase to avoid squashing all of the history; each commit is independent.