Clear up separation of Si, Si-Derived and Metric Units

[atmoos]

Celsius currently implements IMetric, even though it is formally an si-derived unit. As it requires a transformation, we cannot implement it as an ISiUnit and ISiDerivedUnit has been deleted. This poses a dilemma: formally it is an si-derived unit, that currently "needs" to be implemented a metric unit.

Find a way to resolve this:

• Re-introduce ISiDerivedUnit.
• Let SI units also implement a transform function (ugly?)
  • relates to Resolve the multitude of Measures implementations #4
• ...