[racket] Numbers with dimensions

Laurent writes:

 >     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.
 > 
 > The units are now in a separate file but I did not try to build another base.
 > But at least it's possible to require only measures/measure and measures/convert
 > and define one's own set of dimensions and units.

That looks good. When I find some more time (meaning: probably around
Christmas), I'll play with this and try to make a database for
atom-scale units, which is what I use all day.

Konrad.