Arithmetic with affine units results in rational when integer would suffice

When adding a temperature difference in `K` to a temperature in `°C`, one often wants to have the result in `°C` as well. `ContextUnits` can be used to achieve this.
```julia
julia> x = 5 * ContextUnits(°C, °C)
5 °C

julia> x + 3K
8//1 °C
```
However, the result has numerical type `Rational{Int}` even though `Int` would be just fine. The reason for this is that the `+` method in this case converts both quantities to `K` to add them before converting the result back to `°C`.

Start by tracing the + method used for ContextUnits and the conversion back from K to °C. Reproduce the shown Julia example and inspect how numeric types are preserved during affine-unit arithmetic. Done means adding 3K to 5 °C produces 8 °C with an integer numeric type rather than Rational{Int}.

Written by the indexing model from the issue text.