Fix a bug in pvtu2vtu tool

@MaxDoe reported an issue concerning the tool pvtu2vtu with the Konates model

For the tool a fast point search is required. This point search is supported by an OctTree data structure. In order to find a point p in the OctTree a vector of point candidates in a small interval/range around p are returned. If the range is empty, i.e., the lower and upper range boundaries are the same, the OctTree range query can not return any points. The Konates model contained such a special case.

The solution to this is to slightly enlarge the search range in the pvtu2vtu tool.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented? - no behaviour change