Reconcile 2d resolution with isotropic resolution

[uniomni]

GDAL and GeoServer operate with (anisotropic) resolution, i.e. a 2-tuple (resx, resy). However, most raster datasets operated with a common (isotropic) resolution in both directions so a robust strategy is needed to map from (resx, resy) to a single number. Unfortunately, resx and resy are sometimes noticably different. A post was sent to the geoserver mailing list on 30th October 2011.

This is also needed to scale resampled datasets (issue #168)

[uniomni]

This is no doubt linked to issue #102