ENH: Small changes to ase/io/trajectory.py and ase/parallel.py + renaming ase/io/pdb.py

The name clash (see #33 (closed)) between the python debugger ("pdb") and ase/io/pdb.py seems to be the main reason why some python programs are crashing.

Old calculators (e.g. lammpsrun), which are not derived from Calculator, may not have the attribute calculation_required. As a consequence no energies or forces will be written Trajectory files, even if they are available.

world.split() allows do divide one mpi world into several to allow multiple calculators be spawn from one script to work in parallel on a problem(for e.g. neb-parallel runs).

MPI-broadcast has no identifiers. If instances of parallel_function are nested, the outer function in non-root process will receive the result of the inner nested function in the root process.