Add label to reach point

[sjib]

Description:
In order to solve QGEP/QGEP#9, a first step is to add a _label to reach point. The update procedure is entered in the update process of the wastewater structure labels. I tested the section for reach_points and for wastewater_structures separately, and they work. However, putting the code into one function as in the push probably causes recursions. Comments and ideas appreciated.

Related issue:

• Labeling of Manholes and Special Constructions

Related pull request:

• Add label to reach point