feat(units): Add dimensionless units

[adrfantini]

This PR adds dimensionless units, in particular %, pu, ppm, ppt, ppb and ppq.

These are all very useful units both in the energy and chemistry sectors.
See here and here for references.

I am not entirely sure if this risks breaking other things (especially %?); please LMK if there's something I should improve.

[gwhitney]

Correct, the situation with % is much more involved than this brief PR, although that is a small part of it, see #3590. In any case, closing this PR as it cannot be acted on until #3590 is decided on, and then how such units should be handled will depend very much on how Jos decides on #3590. For example, there is the question of whether addition with ppm units should work as with percent: 2 + 10% is currently 2.2, so should 2 + 100 ppm be 2.0002?