Mesh Validation Margins

This adds margins to the mesh validation methods for detecting collapsible nodes and "zero volume elements". For both instances thresholds can now be specified to also be able to test for nodes within a given distance and elements with a volume below a given threshold.
Also, collapsible node ids are now displayed in the data explorer dialogue.
It's correct ... the min volume a mesh element needs to have (given by min_volume) so it is not considered faulty.
I'll change the wording a bit so it's easier to understand at first glance.