request addition of Rankine and/or temperature difference units

[tara_athan]

When temperature data is expressed as a difference (e.g. deviation from an average) the units are on a ratio scale. So to convert a Celsius temperature difference to a Fahrenheit temperature difference, we would just multiply by 9/5, rather than using the affine function. This could be accomplished by using Kelvin and Rankine as the units. Alternatively, a synonymous unit of something like d_Cel and d_Fahr could more clearly state the distinction.