simplified surface extraction method
implemented calculation for edge length range inside mesh class
simplified edge length comparison
Fix for finding eigen on Windows when installed devguide-like.
changed initial max value to negative double max
Merge pull request #322 from rinkk/MeshEdgeRangeCalculation

Edge length range calculation inside Mesh
fixes based on PR comments
Merge pull request #321 from rinkk/MeshSurfaceExtractionSimplification

Simplified mesh surface extraction
Small fix for yesterdays doxygen fix.
added methods to each element class that test if an element is valid
moved the 4-points-on-a-plane-check into AnalyticalGeometry
fixed typo
added optional parameter for checking zero volume of elements
Merge pull request #324 from rinkk/ValidElementTesting

Testing mesh elements for geometric correctness
Merge branch 'master' of https://github.com/ufz/ogs into petsc_vector