\pu: Wider spacing between digit groups

[quasicomputational]

Currently, the thousands separator added by \pu is 2 mu = 1/9 em wide, and looks like this:

I think that's too narrow: I can't pick the thousands apart at a glance. It's even worse in smaller sizes - even looking quite closely, I was barely able to tell where the spaces between groups were and what was just a wide-looking kerning pair.

The Unicode character usually used for this is either U+2009 THIN SPACE or U+202F NARROW NO-BREAK SPACE, but that's not applicable to mhchem (it doesn't insert a character, but instead formats with spaces). Those glyphs are typically 1/5 to 1/6 em wide, which is significantly wider. Here's a mock-up of what 3 mu = 1/6 em looks like:

For me, that's much better!

[mhchem]

Looking at your images, your judgement makes sense. In particular the 2/4 combination is tricky. I most likely will increase the spacing in one of the next updates. (Currently, I am waiting for feedback on my TypeScript implementation. Hoping this will be the only codeline to maintain in the future.)