MeshQualityController

Adds a MeshQualityController class to OGS to call mesh analyis and correction functionality.
Currently this includes:
* finding unused mesh nodes (moved here from Mesh-class)
* removing unused mesh nodes (moved here from Mesh-class, this is no longer default functionality when a mesh is loaded)
* analysis all mesh elements (using the functionality from PR #324)
* output of mesh analysis results (currently used for output in GUI but this returns a string and can be output in console just as well).

The ```isValid()``` method in element classes has been expanded to return error-codes as bitsets, allowing to check what specifically is wrong with a mesh elements instead of just returning true/false.