Add St_Angle/St_azimuth function：
St_Angle：
Enter three point, which represent two intersecting lines. Returns the angle between these lines. Point 2 and point 1 represent the first line and point 2 and point 3 represent the second line. The angle between these lines is in radians, in the range [0, 2pi). The angle is measured clockwise from the first line to the second line.
`
mysql> SELECT ST_Angle(ST_Point(1, 0),ST_Point(0, 0),ST_Point(0, 1));
+----------------------------------------------------------------------+
| st_angle(st_point(1.0, 0.0), st_point(0.0, 0.0), st_point(0.0, 1.0)) |
+----------------------------------------------------------------------+
| 4.71238898038469 |
+----------------------------------------------------------------------+
1 row in set (0.04 sec)
`
St_azimuth：
Enter two point, and returns the azimuth of the line segment formed by points 1 and 2. The azimuth is the angle in radians measured between the line from point 1 facing true North to the line segment from point 1 to point 2.
`
mysql> SELECT st_azimuth(ST_Point(0, 0),ST_Point(1, 0));
+----------------------------------------------------+
| st_azimuth(st_point(0.0, 0.0), st_point(1.0, 0.0)) |
+----------------------------------------------------+
| 1.5707963267948966 |
+----------------------------------------------------+
1 row in set (0.04 sec)
