Feature replica system

[lohedges]

This PR adds a new internal ReplicaSystem class to aid rapid setup and tear-down of replica exchange simulations. This works by storing a single system for the shared topology and a trajectory iterator for the coordinates of each replica. With this approach it is possible to use the native parallelism of the Sire trajectory interface to rapidly read and write frames for each replica. The design also allows for large data compression when passing information between stages of a replica exchange workflow. Instead of passing multiple stream files (one for each replica) the user can now pass a single stream file and trajectory.

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