Backsubstitution

[dawnandrew100]

Resources:

This is a C implementation https://github.com/DmitriBogdanov/Householder-QR/blob/master/source/thirdparty/Eigen/src/Eigenvalues/EigenSolver.h#L458

I want to see if my current back-substitution function is of any use in this case or whether I'll need a brand new function just for this