Simplification for quantities

[sharkdp]

A few examples (X = unit that does not have dimension length):

50 cm/m   => 0.5        (already supported)
50 X*cm/m => 0.5 X
1 mm/m    => 0.001      (already supported)
1 X*mm/m  => 1mX        (adapted SI prefix)

Derived rules:

• If a whole physical dimension can be canceled out (length above) => remove all units of this dimension and transfer the SI prefix to the new units (and/or re-scale the numerical value)

(WIP)