Arbitrary path connectivity in OpenMM

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Plain path integral molecular dynamics calculations typically involve sampling of regular paths that map directly onto atoms ("ring polymers" at finite temperature, "linear polymers" in PIGS). There are situations that arise in some approaches (e.g. partial particles in the worm algorithm and broken paths for efficient estimation of off-diagonal operators) that require the ability to sample paths that do not correspond to atoms in the usual sense. We would like to add to OpenMM the ability to manipulate paths in path integral simulations in arbitrary ways (break them up, reconnect them, etc.) to allow for more general sampling.