physical_quantities: treat the angle as a dimension (radians) #22

Problem / use case

Radians are dimensionless in physical_quantities, so conversions such as 1 Hz ↔ 2π rad/s aren't recognised. compareExpressions' test_radians_to_frequency is xfail on every version.

Proposed solution

Treat the angle as a base dimension, as #1 did. #1 was closed without merging, so this issue tracks reopening that work or redoing it on the current code.

Alternatives considered

Special-casing rad in the conversion code.

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