Unify save_trajectory, savecoords and writechemfile

[axsk]

We currently have three different pdb writers (OpenMM, mdtraj and chemfiles).

All of them have different characteristics regarding performance and compability, so I would like to keep them around.
However, the current API is a mess, this should be cleaned up.

It would make sense to look how similar packages handle this, especially with a regard to FileIO.jl

[axsk]

see #33