Temperature standard arithmetic (WIP)

[angularsen]

Do not merge.

This change is still up for debate. See discussions in #518, #550 and #560.

Created this PR just to have all the work of #550 and #560 in one place in case we want to standardize temperature arithmetic later. We decided to postpone this for v4 as we couldn't agree on whether this change was for the better or not.

• Generate default arithmetic (no special case returning TemperatureDelta)
• Remove Multiply() and Divide() methods
• Remove custom - operator overload
• Add Temperature.ToDelta(other) instance method
• Add TemperatureDelta.FromTemperatures(left, right) static method
• Create tests for addition and subtraction of Temperature
• Fix arithmetic code generation for all quantities, it produced wrong results for Temperature and would also be wrong if any other quantities had units that have different zero values (ax + b linear relationship instead of just a factor conversion ax).

Fixes#518

[angularsen]

This text was removed from 3.x to 4.x migration wiki page and should be added back to a future migration page if we do complete this PR:

Change Temperature arithmetic back to use base unit Kelvin

Instead of a special case for Temperature, we now have the exact same arithmetic as for other quantities.
Previously subtracting two temperatures would return a TemperatureDelta. Instead, you can now add and subtract two temperatures and they will return a new Temperature with addition/subtraction performed in base unit Kelvin while maintaining the left hand argument's unit.

Temperaturecelsius30,celsius20;// Assume assigned..// Beforevarx=celsius30+celsius20;// Not possibleTemperatureDeltadeltaCelsius10=celsius30-celsius20;// delta 10 degrees Celsius// After - this can get confusing if you are working with Celsius or FahrenheitTemperaturecelsius323=celsius30+celsius20;Temperatureminus263Celsius=celsius30-celsius20;// For addition/subtraction when working with Celsius or Fahrenheit, you probably want to use TemperatureDeltaTemperatureDeltadeltaCelsius20=TemperatureDelta.FromDegreesCelsius(20);Temperaturecelsius50=celsius30+deltaCelsius20;Temperaturecelsius10=celsius30-deltaCelsius20;// Or use these convenience methodsTemperaturedeltaCelsius10=celsius30.ToDelta(celsius20);TemperatureDeltadeltaCelsius10=TemperatureDelta.FromTemperatures(celsius30,celsius20);

[angularsen]

Closing this. We can keep the branch alive for some time and can always reopen the PR later.