Add swarm simus to work queue directly when a restrained simu has finished

[oliverfleetwood]

Now we perform the string method one step at the time. For example, all grompp calls have to finish before any mdrun call starts. All restrained simus need to finish before any swarm simu starts. It would be more computationally efficient if we add the new jobs to the work queue as soon as the blocking step has finished. In my plumed-based version of the string code I managed this by having a Job class that the queue handles. The job has a list of jobs that should be sumbitted once it's finished. So the StringRunner would conifgure all jobs and their dependencies, then invoke the Master-Slave MPI wokers only once per iteration.

This is especially useful if the number of points on the string is not divisible by the number of worker MPI ranks.