I've had a hard time understanding the `which` parameter of ARPACK, so
I'm submitting this patch with the hope that it becomes clearer.
* Eigenvectors have all norm 1, thus the parameter `which` is
applied to eigenvalues and not eigenvectors.
* Clarification on what magnitude means in the context of
* Clarification on what 'LA', 'SA' means. ARPACK docs refer to them
as Largest/Smallest Algebraic value: those are real numbers with
its respective sign (not in absolute value). I could not understand
what was meant before as `Largest Amplitude`.