chi_angles_mask
============================

If there is a mask of corresponding torsion angle in the amino acid. A two-dimensional array, the length of first dimension is 20, representing 20 different amino acids, consistent with the order of 'restype_order'. The second dimension has a length of 4, and each amino acid has a maximum of four side chain torsion angles, with a value of 1.0 indicating the existence of the torsion angle and 0.0 indicating the absence of the torsion angle.

+--------+-------+-------+-------+--------------+
| value  |       |       |       | amino acid   |
+========+=======+=======+=======+==============+
| 0.0    | 0.0   | 0.0   | 0.0   | #ALA         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 1.0   | 1.0   | #ARG         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #ASN         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #ASP         |
+--------+-------+-------+-------+--------------+
| 1.0    | 0.0   | 0.0   | 0.0   | #CYS         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 1.0   | 0.0   | #GLN         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 1.0   | 0.0   | #GLU         |
+--------+-------+-------+-------+--------------+
| 0.0    | 0.0   | 0.0   | 0.0   | #GLY         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #HIS         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #ILE         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #LEU         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 1.0   | 1.0   | #LYS         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 1.0   | 0.0   | #MET         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #PHE         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #PRO         |
+--------+-------+-------+-------+--------------+
| 1.0    | 0.0   | 0.0   | 0.0   | #SER         |
+--------+-------+-------+-------+--------------+
| 1.0    | 0.0   | 0.0   | 0.0   | #THR         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #TRP         |
+--------+-------+-------+-------+--------------+
| 1.0    | 1.0   | 0.0   | 0.0   | #TYR         |
+--------+-------+-------+-------+--------------+
| 1.0    | 0.0   | 0.0   | 0.0   | #VAL         |
+--------+-------+-------+-------+--------------+