Add averaging of force and torque matrices over timesteps

[harryswift01]

Currently, the force and torque matrices are being replaced with values from the latest frame, rather than maintaining a running average across timesteps. This leads to loss of temporal information and less representative data.

• Implement running averaging of force and torque matrices over timesteps, following the approach used in waterEntropy
• Change the default grouping to be by molecule instead of by individual atoms.
• Ensure that water molecules are excluded from the grouping process.