Handle RESX and RESY parameters properly

In a GetCoverage request of a web coverage service, we can specify either width/height or resx,resy.
In our current implementation this condition is tested, but the behaviour of resx/resy is the same as the one for width/height.

We need to implement these parameters properly. According to the WCS 1.0.0 standard:
"The parameters RESX and RESY define the grid-cell size along the first and second axes of the coordinate reference system given in CRS or RESPONSE_CRS."