I've pushed some prototype code to a new branch: https://github.com/maths/moodle-qtype_stack/tree/ATUnitsNum
This implements new answer tests which combine support for scientific units with two more numerical answer tests, NumAbsolute and NumRelative. On reflection, I'm not sure how useful UnitsAbsolute will actually be in situations where the student has compatible units. See the docs for an explanation of this.
I've not added in the "decimal places" answer test, as significant work needs to be done on the underlying numerical test before this is truly reliable. In particular the decimal places test is implemented, and the code works, however it cannot be used in STACK because of difficulties with validation. To fully implement this we need an "ephemeral form" for floating point numbers, which will require some more work.
I'd really like to receive some test cases from science teachers to test this, particularly in edge cases involving awkward numerical rounding. This is not my area of expertise, so comments and tests very welcome. When people are happy with this I'll merge this back with the master branch for release.