Altered WCS link generation method width/height

Fixes #1356; WCS links should now return their native grid height and width
based on the WCS describeCoverage spatial grid.
Also changed how links are generated so that when fetching URLs on
very coverages the server does not need to issue several getCoverage for the
 entire grid extent of the coverage, which would ordinarily bog down the server
for large coverages.
Merge pull request #1373 from benjwadams/1356-fix_wcs_link_raster_resolution

Altered WCS link generation method width/height