Add Temperature Units

Adds unit conversions for Celsius and Fahrenheit, requiring the ability to define units where 0 does not represent the absence of the quantity (E.g. 0 Celsius is equivalent 273.15 Kelvin ).

Additional "delta" units are also added to represent relative differences in thermal energy, and do not include the offset zero. For example, an increase of 10 degrees C is equivalent to 10 Kelvin, even though 10 C as an absolute measurement is 283.15 K.