Support differences vs absolute values
Sometimes conversions along a dimension represented by an units::OffsetRule
differ in how they should be applied depending on whether the input value is absolute or relative to some reference point.
An example of this is temperature scales: when converting an absolute temperature from Celsius to Kelvin, the offset should be included. But when converting temperature differences, the offset should be ignored. This is currently handled by testing whether the units of the measurements to be converted are solely temperature or whether they include other units (such as heat conduction coefficients). In general, it would be better for the units library to be explicit about whether input values are absolute or relative.