1. Information about a unit's dimension now uses inheritance.
_m is an instance of DistanceAlias, which is derived from Alias.
A UnitNode now keeps a pointer to an Alias and one to a Prefix.
All aliases are still defined as constexpr.
This cleans up a lot of the code used namely for computing the
additional outputs in Calculation.
2. Instead of being defined with a string, each unit is described by its
ratio with the base SI unit (ex: _L is 0.001 instead of "0.001_m^3").
This greatly speeds up the calculations using units, as the algorithm
to find the best unit used to parse the definition.
Change-Id: I4d6ed6ad4cb967026a3f01a335aec270066e2b9f
Now, Decimal node "1.000E3" is not equal to Decimal node "1E3" node anymore.
This prevents from using double comparison (which is sometimes wrong due to
double precision) to implement decimal comparison.
