Deprecate and remove `net_force`, `net_torque`, `net_energy`, and `net_virials`

[joaander]

Description

• Deprecate and remove net_force, net_torque, net_energy, and net_virials from local snapshots.

• Check whether *_local_force_arrays recomputes after a set_snapshot

• Add a new snapshot_set signal. Use this to test when to recompute forces. Local snapshots should send the signal when exiting the context manager. Also document that changes to particle energies will not be reflected until the context manager exits.

• Similarly document that ThermodynamicQuantities computes based on the content in the system state last set by the integrator and that snapshot modifications will be ignored.

Motivation and context

These quantities are not part of the system state and never should have been added. Many users reference these, but find it confusing when they do not update as expected. In reality, these are internal quantities computed by Integrator when needed to integrate the system forward in time.

Users should access the Force properties to obtain these quantities. If the user has multiple Force objects, it is the responsibility of the user to add them together when needed.

[joaander]

These are needed for Python implemented half step hooks: #1831