[GEOS-6688] WCS 1.1.1 GetCoverage request fails...

Edited the WCS 1.1.1 class to let cubic (as bicubic) and linear (as
bilinear) be used as valid interpolation methods. Those were disabled in
code and GeoServer would serve an Exception when trying to access those
interpolations.

I've also added prelimenary support for bicubic and bilinear as keywords
and made a very small performance improvement on GetCoverage requests.
This change is breaking all the tests making use of schema validation. Maven runs tests from the module's directory, not from the root (even when doing a full build)