Add a new metric computation for level set, simply based on ls gradient and...

Add a new metric computation for level set, simply based on ls gradient and hessian eigendirections.
meshAdapt was modified to receive and intersect multiple metrics, preserving the most anisotropic metric.
Also added input and output functions to dgDofContainer, for the "NodeData" format.
Benchmark for anisotropic adaptation to level set iso-zero created in benchmarks_gmsh/LSIsozeroAniso.