Only mark cells if subdomains have set in the geometry.
Update some comments.
Fix issue 153: Wrong result for TetrahedronCell::squared_distance
Use boost::nth_element throughout geometry code
Merge branch 'logg/fix-nth-element'
Merge branch 'benjamik/fix-csg-cell-marking'
A hack to sort 1D data into coordinate order
Merged master into chris/fix-issue-59
Minor formatting fixes.