Allow different ordering methods for Sparse solvers

[jnbrunet]

At the moment, sparse solvers using Eigen as backend use the default Eigen::AMDOrdering ordering method. While this method performs well for unstructured meshes, it does worse for structured grid (such as the regular grid). We should be able to change the ordering method to the identity.

In addition, Eigen provides a wrapper to METIS ordering algorithms. It would be a good occasion to add it to the list of supported ordering methods.

Example:

root.addObject('LDLTSolver', backend='Eigen', ordering='None')
root.addObject('LDLTSolver', backend='Eigen', ordering='AMD')
root.addObject('LDLTSolver', backend='Eigen', ordering='METIS')