Handle Celsius/Kelvin and other +/- delta unit conversions

[xoofx]

kalk is supporting complex conversions with multiple units, but only if they are multiplied or divided or powered.

But today, kalk is not able to express simple conversions like:

[°C] = [K] − 273.15

The code involved is in KalkExpressionSimplifier.cs is relatively basic to simplify units operations (e.g C * km / h can be reduced to C * m / s) but I'm not sure on how to handle the +/- delta easily.

For example a unit °C/m, how would be expressed the right side? (K-273)/m?