center of mass r0 of the group of atoms is computed,
  then a restoring force = -k*(r-r0)*mass/masstotal is applied to each 
  atom in the group where mass = mass of the atom and masstotal = mass of
  all the atoms in the group - thus &quot;k&quot; should represent the total
  force on the group of atoms (not per atom)
for springforce style, a xyz position of NULL means do not include that
  dimension in the distance or force computation
for dragforce style, apply a drag force of magnitude f to each atom in