Improve/correct unit validation in schema

[tsalo]

Once #885 is merged, value formats will validated (or at least validate-able) by the schema. However, the unit format will allow any string, which does not match the actual description of valid unit values.

From @yarikoptic:

I thought we had discussion and there was a decision on some unicodes to be ok (but not any), then with #411 CMIXF-12 ascii recommended. https://github.com/bids-standard/bids-specification/blob/HEAD/src/99-appendices/05-units.md mentions µ (could be only as prefix) Ω (only as suffix) and ° (all by itself?). So may be regex and test here could be made more stringent? well, aforementioned page gives example just as "NOT RECOMMENDED" instead of "WRONG": volts per microsecond so may be my whining is all in vein? But then I would restrict to composition of alphanumerics, space, - and those 3 unicodes, and thus disallow everything else.

This stems from #885 (comment).