Make quantities independent of units

[atmoos]

The concrete quantity structs would ideally not have any dependencies to any concrete units, only to unit interfaces.

I have no clue how to accomplish as this collides square on with #6 . Possibly a very interesting design problem to work on.

[atmoos]

I think one way of accomplishing this without sacrificing #6 would be to keep the seven SI base units in the core library. (it's kind of justifiable.

With these the base SI units for all types of quantities can be constructed. These could, perhaps even should, then be the basis for additive and/or multiplicative identities.