added a base class to MeshLayerMapper and LayeredVolume for handling basic functionality and class structure

reworked prims-layer-generation to work from top to bottom and prefer older (lower) layers in cases of conflicting information

Replace BaseLib::number2str with std::to_string.

[GeoLib] Another version for GEOObjects::getGeoObjects().

The commit is a preparation to access GeoObject objects via the geometry
name and the object name (patch name).

Also remove the header StringTools.h everywhere.
Add a previously missing sstream include in DateTools.cpp.

[bugfix] fixed issue where raster to mesh conversion would fail if all all intensity values were negative

Cleanup

Radius edge quality metric for meshes

This is required for an upcoming application.
It was already discussed in #89 and will come up
with more changes as the application grows. So
step by step all #includes will have the full
paths prepended.

Minimal bounding spheres for points