Moved to TimeData and FixedData as basic data structures.
Modifications in TimeData and trajectory to allow the use of either shape and dtype or data.
Update shortened attribute s,t,v to step,time,value.
Removed erroneous checks on shape/dtype/data in particle_data.
Addition of missing docstrings.
Added time independent particle data in example run_h5md.py
Change of argument name for more generality.
Change /trajectory to /particles