[Bug]: complex number forces produced

[jkalayan]

Bug Report

When combined_forcetorque and customised_axes are True, complex numbers are produced for some translated forces, for molecules that are one united-atom in size/ not covalently bonded to other united atoms.

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To Reproduce

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YAML configuration

run1:
  
  top_traj_file: ["~/GAFF/hydrogen-sulphide/molecules.top", "~/GAFF/hydrogen-sulphide/trajectory.crd"]
  force_file: "~/GAFF/hydrogen-sulphide/forces.frc"
  file_format: "MDCRD"
  end: 1
  temperature: 213
  water_entropy: False
  kcal_force_units: True
  combined_forcetorque: True
  customised_axes: True

Actual output

CodeEntropy/CodeEntropy/levels.py:421 in get_weighted_forces 
                             |   418 │   │                                                                                                             │
                             │   419 │   │   for atom in bead.atoms:                                                                                   │
                             │   420 │   │   │   forces_local = np.matmul(trans_axes, data_container.atoms[atom.index].force)                          │
                             │ ❱ 421 │   │   │   forces_trans += forces_local                                                                          │
                             │   422 │   │                                                                                                             │
                             │   423 │   │   if highest_level:                                                                                         │
                             │   424 │   │   │   forces_trans = force_partitioning * forces_trans   
UFuncTypeError: Cannot cast ufunc 'add' output from dtype('complex128') to dtype('float64') with casting rule 'same_kind'

[skfegan]

The complex number error also happens when combined_forcetorque = False and customised_axes = True. But the code runs (at least for 20 frames) when customised_axes and combined_forcetorque are False