OpenMM script builder plugin

[badarsh2]

Adapted from http://builder.openmm.org
In addition to the script, coordinate file is also generated if the format is supported.

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Signed-off-by: Adarsh Balasubramanian badarsh2@gmail.com

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[ghutchis]

@cryos - just checked with the current CI and this builds cleanly. I'd like to merge this:
https://github.com/ghutchis/avogadrolibs/actions/runs/90434567

[cryos]

Not had a chance to put it through its paces, but clean build, and it would be great to use OpenMM more.