QB4ST - Spatio-temporal extension to RDF-QB

I have created an initial set of specialisations of RDF-QB elements to
support an interoperable means of describing spatio-temporal
characteristics of dimensions and measures in metadata about data sets.

There is a wiki page with some introduction to RDF-QB here:
https://www.w3.org/2015/spatial/wiki/RDF_Datacube_for_Coverages

and the ttl is here:
https://github.com/rob-metalinkage/sdw/blob/gh-pages/coverages/qb4st/qb4st.ttl

(and i generated a pull request).

Requesting a sanity check ASAP - and feel free to educate me on how such
things _ought_ to be defined - first step is to outline what is required
and what it needs to do - then we can refine the logic and language.

Cheers
Rob Atkinson