Probe closest node

This is a "quick-and-dirty" solution to make PViewData::search{Scalar,Vector.Tensor}WithTol() return the value at the closest node if the search point is not found, e.g. if it is just outside of the view data.

This should probably be improved: the current implementation has a linear complexity in terms of the number of nodes in the view (we could use a kdtree e.g. from flann instead; however the construction cost might be prohibitive if there are only few queries outside the octree), and it is placed inside a #pragme omp critical because PViewData iteration is not thread-safe (due to the internal cache).