[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.
Indeed from the IDE. I'm usting IntelliJ IDEA Ultimate 13.1 on Windows. I am not sure what specifically causes the issue, but thanks for considering a change. Maybe a path check with a fallback could work?