Calculate the chi1 torsions of a trajectory. chi1 is the first side chain torsion angle formed between the 4 atoms over the CA-CB axis.
| Parameters: | trajectory : Trajectory 
 opt : bool, default=True 
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| Returns: | indices : np.ndarray, shape=(n_chi, 4) 
 angles : np.ndarray, shape=(n_frames, n_chi) 
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