Modified SequentialQR so that you can pass it a matrix Y and it will compute Q^TY and R instead of Q and R.
This is useful because Q^TY can fit in memory even if neither Q and Y can, so SequentialQR will do no additional IO other than loading the matrix (in particular no writes).
Also added some basis expansion stuff; idk how useful that is without splines.
Ridge and splines coming soon.