fixed issue where elements would be constructed wrong if layer differences of one element where larger than 1

Conflicts:
	MeshLib/MeshGenerators/LayeredVolume.cpp

Conflicts:
	MeshLib/MeshGenerators/LayeredVolume.cpp

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

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

http://www.viva64.com/en/d/0254/print/

Check only if there is a neighbor.