[with patch] Matrix.minors

[malb]

The attached patch implements a method to return the list of all k-minors of a matrix A.

Let A be an m x n matrix and k an integer with 0 < k, k <= m, and k <= n. A k x k minor of A is the determinant of a k x k matrix obtained from A by deleting m - k rows and n - k columns.

The returned list is sorted in lexicographical row major ordering, e.g., if A is a 3 x 3 matrix then the minors returned are with for these rows/columns: [ [0, 1]x[0, 1], [0, 1]x[0, 2], [0, 1]x[1, 2], [0, 2]x[0, 1], [0, 2]x[0, 2], [0, 2]x[1, 2], [1, 2]x[0, 1], [1, 2]x[0, 2], [1, 2]x[1, 2] ].

Note I am not sure if this method is too trivial or too specialised to be included with SAGE. I am submitting it here such that others can decide on that.

[was]

This should definitely go in.