Add variables, argument parsing and h5md elements creation for velocity, force and species
Refactor pack loop
Add pack and write for velocity, force and species
Force packing and unpacking of all existing elements at each dump.
Else, the buffer has a varying size depending on the timestep and this
breaks the data collection.

TODO: make this more efficient, if possible. General dump code might prevent that, however.
TODO: a bug appeared when running in parallel.
Fix k_species count (dimension is always '1' and not 'dim').
Add link argument to h5md_create_box.
Include box data in the situation where there is no position.
Update documentation of h5md dump style.