Add molecular aligning external field

[DanqiLANG]

Summary

This PR adds an external aligning field for molecular Monte Carlo simulations, using the molecular orientation definitions introduced previously.

For each molecule, the field contributes

[
U_h = -\mathbf h \cdot \mathbf n,
]

where (\mathbf n) is the previously defined unit molecular orientation and (\mathbf h) contains the field direction and coupling magnitude.

The initial target is the three-site ortho-terphenyl model using ordinary particle-displacement moves.

Changes

• Add external-field types:

  • ExternalField
  • NoExternalField
  • AligningField

• Add field-energy functions:

  • field_energy
  • total_field_energy

• Include the external-field contribution in the initial molecular Hamiltonian.

• Include the field-energy difference during ordinary particle displacement:

  [
  \Delta U_h =
  -\mathbf h\cdot
  \left(\mathbf n_{\mathrm{new}}-\mathbf n_{\mathrm{old}}\right).
  ]

• Add TOML configuration support.

• Add architecture notes and a minimal single-molecule ortho-terphenyl validation example.

[V-Francois]

Looks good.

There are merge conflicts though. I think it's because you started this branch from the previous PR's branch (which has been merged since), rather than from main after the merge.
Can you rebase onto main to include just the new changes (the fields, and not the orientation that are already merged).