prefer unit symbols to unit names

[crotwell]

See this related issue in stationxml documentation:
FDSN/StationXML#76 (review)

Unit names vary from language to language, while the SI unit symbols do not. For example in the US, m is meter but is metre in other English speaking countries and in the official SI brochure:
https://www.bipm.org/en/publications/si-brochure

And of course longer unit names do not really add anything in most cases, "meter / second" is not really any clearer than "m/s". If a longer string is needed for presentation purposes, it can be put in the description.

Propose adding to wiki
https://github.com/iris-edu/stationxml-validator/wiki/Unit-name-overview-for-IRIS-StationXML-validator

SI symbols are preferred over names to avoid language differences, for example `m/s` is preferred over "meter/second" or "metre/second"

Related first rule, in SI "m/s" is a unit "symbol" not an abbreviation. So

Use SI units, base or derivative, and their standard symbols whenever possible. Spell out and avoid abbreviations of non-SI units.

Maybe add the SI brochure link to bottom of page too.