Support relative and absolute temperatures with is_equivalent

`is_equivalent` should return True for these two generics:
```
<GenericCompositeDimension: (LengthUnit^2) * MassUnit * RelativeTemperatureUnit / (TimeUnit^3) / AbsoluteTemperatureUnit>
<GenericCompositeDimension: (LengthUnit^2) * MassUnit / (TimeUnit^3)>
```