Successful GDP processes don 't return output

Running the example scripts results in no output being written even though the GDP process was successful. The XML for output from the GDP request are not correct. An API change for the wps.execute() call in the OWSLib library is the cause of this problem (https://github.com/geopython/OWSLib/issues/536) where the output argument changed from a string to a tuple. Changes need to be applied to the submitCustomBioclim(), submitFeatureCategoricalGridCoverage(), submitFeatureCoverageWCSIntersection(), submitFeatureCoverageOPenDAP(), and submitFeatureWeightedGridStatistics() routines.