[GEOS-6809] WCS 2.0 DescribeCoverage should be able to handle single granules

Please look at the following JIRA: https://jira.codehaus.org/browse/GEOS-6809. Note that the following pull request will move 2 classes (SingleGranuleGridCoverageReader.java and GranuleCoverageInfo.java) from the wcs2.0-eo module to the wcs2.0 one.
They are not generic extensions, they are describeCoverageExtensions