CAS-14283: Add _gclean.mustache
Added the mustache template for _gclean. This commit contains the mustache
template as well as the generated python file. The generation happens via
Darrell's expandtemplates script within casagui, and was run offline prior to
this commit.
It might be necessary to incorporate some version of this file into the build
system to generate the .py files on the fly, but for the moment this seems to
work alright.
