Recover ability to run assembly with plant.assembly

[dfalster]

These updates

• move functions to more suitable homes
• update functions to work with new interface
• add some tests

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (develop@cd20ab2). Click here to learn what that means. Patch has no changes to coverable lines.

:exclamation: Current head da64d00 differs from pull request most recent head 91d66a7. Consider uploading reports for the commit 91d66a7 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #370 +/- ## ========================================== Coverage ? 82.78% ========================================== Files ? 99 Lines ? 8962 Branches ? 0 ========================================== Hits ? 7419 Misses ? 1543 Partials ? 0 ```

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