utility functions to collect the paths of prev_state files for completed iterations.

[eirrgang]

Create a brer.utilities module. Provide utility functions for collecting and managing the currently available samples of the conformational ensemble.

• Gather trajectory outputs from completed iterations from which to construct the conformational ensemble.
• Construct a new simulation input from an arbitrary element of the prev_state collection. Determine the working directory path and return a new SimulationInput with the checkpoint file from the previous production work dir.

Requires #23