separated inverse code into an independent file.
Implementation of rank estimation using Gauss elimination.
Some rewrite of LU  decomposition. Early code of crout method.
Introduced functions like permuted_rows, permuted_cols, scale_row or column for lower or upper triangle. scale_rows scale_columns. with corresponding unit tests.
a function to detect if a vector is a permutation vector.
Some performance optimizations of LU decomposition.  Computing the inverse permutation of a permutation vector.
Correct implementation of LU decomposition using Crout method.  Some methods added to Matrix.