ENH: Add velocity reading and writing to read_vasp and write_vasp
Add velocity reading and writing to read_vasp and write_vasp, respectively.
- Reading: After reading positions, if next line starts with 'c' or 'k' (cartesian) read in velocities similarly to positions (see definition of POSCAR file for MD: https://www.vasp.at/wiki/index.php/POSCAR)
- Writing: If velocities are not None write velocities in POSCAR format. Format string uses .format() convention.