mathlib3

46fc73a3 - feat(linear_algebra/matrix): inverse of block triangular is block triangular (#16150)

feat(linear_algebra/matrix): inverse of block triangular is block triangular (#16150) The inverse of a block triangular matrix is block triangular. I took the opportunity to refactor the proof about determinants of block triangular matrices as well. Co-authored-by: Eric <ericrboidi@gmail.com> Co-authored-by: Alexander Bentkamp <bentkamp@gmail.com>