Linearalgebra

I readded several matrix/linear algebra functions that I was missing in the current version and that I had implemented in a branch that I forgot about.
For one thing I added ```matrix.det``` which is very important imho.

Also I added various decompositions (QR, LU, cholesky, Hessenberg, TD, Hessenberg-Triangular), added them to the documentation in the linear algebra section and added a test file for these particular linear algebra functions.