Composition

[hgrecco]

As I mentioned in #2131, one thing that I would like to see is moving to a composition architecture. We already did this with delegates. We can do it now with conversions.

The idea is to have a delegate to handle conversion between multiplicative units, non multiplicative units, and so on.

In addition, I would like to see if possible to add a flag to Unit that quickly dispatch. This has several benefits

1. clean code
2. caching
3. simplify tesing some other architectures

This would allow us also to raise an exception when an illegal operation. For example (I am eliding the magnitude of the unit for convenience)

u1 = meter (flagged as multiplicative)
u2 = second (flagged as multiplicative)
u3 = u1/u2 (flagged as multiplicative)
u4 = degC (flagged as offset unit)
u5 = degC / u2 (error)

Does this make sense to you?

ping #2108