Add triangle analysis function

This PR adds 3 new functions for triangle.

```mysql
ST_TriangleSlope(GEOMETRY); 
```
Compute the slope of a triangle in degrees

```mysql
ST_TriangleAspect(GEOMETRY); 
```
Compute the aspect of a triangle Aspect represents the main slope direction angle compared to the north direction.

```mysql
ST_TriangleDirection(GEOMETRY); 
```
Compute the main slope direction on a triangle represented by a linesegment

See #46.