QR factorisation of matrices

[unordinateur]

As discussed in issue #645, here is my code contribution to the project, for implementing QR and RQ matrices factorisation.

Here are 2 suggestions for subsequent additions/modification:

• I implemented two helper functions, flipup and fliplr. They should be moved elsewhere, in another module, because they are of vastly more general usage than my factorisation functions.
• I implemented only QR and RQ factorisation of matrices, but many more type of matrix factorisation are possible: QL and LQ, LU and LDU, eigendecomposition, etc... If somebody want to implement those methods, they should!

[Groovounet]

Thanks for contributing!