Fix feptasks.minimize()'s minimisation_steps argument

[zhang-ivy]

minimisation_steps, an argument to feptasks.minimize() , currently matches the openmm LocalEnergyMinimizer.minimize() argument maxIterations in that when its equal to 0, it will run as many steps of minimization as needed, not zero steps.

minimisation_steps should work like this instead:

• None should be interpreted to run until tolerance is reached
• 0 should be interpreted to run 0 steps

See thread: https://github.com/choderalab/perses/pull/1065#discussion_r948027635

related: https://github.com/choderalab/perses/issues/1083

[ijpulidos]

Even if this is a minor change, this is an API breaking one. I think we should leave it to the next milestone release, where many API breaking changes will happen. We also need a docstring for the HybridCompatibilityMixin.setup method, I'll open a new issue for that one.