Add support for OpenMM MonteCarloMembraneBarostat

[lohedges]

This PR adds support for using the OpenMM MonteCarloMembraneBarostat with Sire dynamics via the surface_tension kwarg or map option. The default settings match those used for SOMD1. Currently the only tunable parameter is the surface tension, which should be fine for the majority of use cases. The functionality will be wrapped in SOMD2 by simply exposing the new dynamics kwarg. By default, this will be None, so a user will need to explicitly set a value to use a membrane barostat (along with a pressure and temperature too).

Closes #313.

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• I confirm that I have added a test for any new functionality in this pull request: [y]
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: [y]
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[tom-potter-cresset]

I've run a quick test on one of our GPCR test systems, and dynamics is working as expected