ACTION-318: Quantity Values and Unit of Measures

Dear all,

Last mail, also just for the record, pull request 907
https://github.com/w3c/sdw/pull/907 implemented ACTION-318
https://www.w3.org/2015/spatial/track/actions/318
Write a blurb on how to use unit of measurements with our result class.

This consists in paragraph
http://w3c.github.io/sdw/ssn/#quantity-values-and-unit-of-measures , which
is non-normative, and can be corrected if necessary along with the
examples.

I will send an email to QUDT and OM people to check with them that I
modeled the things right in the examples.

Best,
Maxime