Switch for using immobilisation

If ``Switch for using immobilisation'' is set to TRUE, the position of the center of mass of a set of atoms is constrained by employing a harmonic potential. The usage of this feature is usually necessary if one uses the pulling/stepping mechnism implemented in EGO. For more information see [24,27]. The set of atoms from which the center of mass is calculated and which feel that harmonic potential can be defined by the usuall selection strings defined in Section 4.3. The integration time step at which this harmonic potential is switched on can be defined in `Start integration step for flooding/immobilisation/pulling'. The harmonic constant is set in `Harmonic constant for immobilisation'.