Remove python loop in convex hull filtering

[monodo]

Looks like this is the only remaining operation that is highly time consuming. It's not on top of the todo list...

[monodo]

Also, the call to QGIS processing tools "qgis:..." call script written in python (QGIS 2.18) and are quite slow. For now better use "saga:..." or "grass:..." whenever available. In QGIS3, processing tools are supposed to be rewritten in C++