Approximate SHAKE and RATTLE in LAMMPSEngine.minimize

**Is your feature request related to a problem? Please describe.**
SHAKE and RATTLE cannot be applied in LAMMPS during minimisation. Currently these fixes are just removed during minimisation and reapplied afterwards; however this means that the energy minimisation will be for a different force field than is applied during a simulation.

**Describe the solution you'd like**
Determine a factor by which potential strengths of bonds or bond angles affected by SHAKE or RATTLE are scaled, so that the behaviour of the fixes is replicated by the artificially strong bonds or bond angles. Implement this scaling in LAMMPSEngine.minimize. These parameters will then have to be rescaled to their original values following minimisation.