RestartFileWriter silently drops velocities or forces when only one of the two is present

[galjos]

The atom-line guard requires velocities AND forces, so a system carrying only one of them is written as a bare 6-field position line:

C    0    0    0.0 0.0 0.0

The velocities are gone, no warning is given, and RestartFileReader rejects the file anyway because it expects 12 or 21 fields.