[GEOT-4802] Creation of two new processes called BandMerge and BandSelect. (Backport on 11.x)

Backport of the pull request https://github.com/geotools/geotools/pull/468 on the 11.x branch

Please look at the following JIRA: http://jira.codehaus.org/browse/GEOT-4802.
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