ENH: add description to a trajectory about what created it and how, i.e. an optimization with BFGS.

This enables things such as optimizers and molecular-dynamics method to add a description to a trajectory about itself, i.e. that the trajectory was created using the BFGS algorithm with some parameters such as maxstep. To have the extra description added users need to start using something like:

traj = Trajectory() dyn.attach(traj)

instead of

traj = Trajectory() dyn.attach(traj.write)

The latter still works but will not write the nice extra description.