Allow someone to set the class to use for GeoServer coverage.

[val99erie]

This will allow us to insert a work-around for the problem described in curationexperts/iris#50.

[coveralls]

Pull Request Test Coverage Report for Build 123

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 121:	0.0%
Covered Lines:	1496
Relevant Lines:	1496

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💛 - Coveralls

[eliotjordan]

This seems reasonable. 👍