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

氨基酸中是否存在相应扭转角的mask，二维数组，第一维长度20，代表20种不同的氨基酸，与 `restype_order` 的顺序一致；第二维长度4，每个氨基酸最多有四个侧链扭转角，值为1.0代表该扭转角存在，0.0代表不存在。

+--------+-------+-------+-------+--------------+
| 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         |
+--------+-------+-------+-------+--------------+