/** \brief An example of using the hierarchical factorization of a dense matrix. from .mat file.
* An dense matrix is loaded from "@FAUST_TESTDATA_SRC_DIR@
* \param config_filename : a .mat (MATLAB file) configuration file which contains the parameter of the hierarchical algorithm (default launch with a predefined configuration called hier_fact)
* \param expected_lambda (optionnal) : compared the expected scalar of the factorisation with the computed one in the precision defined with epsilon
*\param epsilon : precision for the test of equality (default value 0.0001)