[racket] Numbers with dimensions

Laurent writes:

 > Pushed: https://github.com/Metaxal/measures#4-dimensions-and-contracts

This is getting better and better every day! The use of contracts looks
like the best way to do run-time dimensional analysis.

My only remaining wish is to be able to define my own set of units and
prefixes, i.e. factor out the second half of convert.rkt. Unfortunately
this doesn't look straightforward, because it must be possible to
use multiple unit systems in the same Racket instance. It might actually
be best to have a specific module per unit system, as there are not
so many useful ones anyway. Each client module would then require the
module with the unit system it needs, i.e. measures/si or measures/atomic.

Konrad.