Eigen
3.4.90 (git rev 67eeba6e720c5745abc77ae6c92ce0a44aa7b7ae)

Get the fillreducing ordering from the METIS package
If A is the original matrix and Ap is the permuted matrix, the fillreducing permutation is defined as follows : Row (column) i of A is the matperm(i) row (column) of Ap. WARNING: As computed by METIS, this corresponds to the vector iperm (instead of perm)