Allow a dynamics object to propagate several independent trajectories

[lohedges]

This PR adds functionality to allow a dynamics context to be re-used for a dynamics block at a different lambda value to the one at which it was created, e.g. within a Hamiltonian replica exchange simulation. Methods are provided to query and set the internal "clock" parameters of the object, as well as swapping the energy trajectory. Building on top of this allows us to extend SOMD2 to enable replica exchange on more modest hardware, where it's not possible to create contexts for all replicas due to memory constraints, or to perform simulations where a very large number of replicas are needed, e.g. ABFE.

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