update on coverages work

Hi all

Here's an update on the three Notes we've been working on.  The aim is to
have them all finalised by end of May, for review and final editorial
tweaks during June.

1. EO-QB

Editor's draft: http://w3c.github.io/sdw/eo-qb/

Dmitry and Sam following up a few remaining issues as listed in
https://github.com/w3c/sdw/pull/742  (that PR has been merged)

2. QB4ST

Editor's draft: http://w3c.github.io/sdw/qb4st/

Main outstanding issue is for me to add an example of how the ontology
would be applied to a simple example coverage.

3. CovJSON

Editor's draft: http://w3c.github.io/sdw/coverage-json/

Outstanding issues:
- Jon to talk to Scott Simmons about options for future standardisation
work on the CoverageJSON format
- Jon to add some more illustrations
- Bill to sort out the references, with advice from Jon where needed
- Jon to tidy section 3.6 (relationship between CoverageJSON and RDF) then
seek feedback from SDW group
- Bill to create cross-references to the BP doc
- Jon to address https://www.w3.org/2015/spatial/track/issues/101

Best regards

Bill