Fix ST_Split function
Fix ST_Split function signature
Add new unit tests
Improve documentation
FIX  description formating and tolerance class variable
Fix ST_Split documentation description
Replace all references centroid by interior
Fix #569
Merge pull request #582 from ebocher/fix/centroid_areasymbolizer

Replace all occurrences of centroid by interior in the GUI.

Fixes #569. Closes #570.
Merge pull request #576 from ebocher/feature/fixst_split

Improve ST_Split

This PR fixes the ST_Split function signatures.
Improves its documentation and uses.
The user can now set a tolerance when a line is split by a point.
Added unit tests.