Added backpropagation of Twiss parameters to the GUI, and fixed the annoying bug in the last row of making the Twiss matrix. Used this in the GUI to implement user-selectable point of setting the Twiss parameters.
Also added various quality of life features to the GUI. Fixed a bug in the initialization of the Twiss model from the GUI, where the initial alpha was always the y alpha.
Also added thin quadrupole and plasma lens element + some refactorization. Added getTransform() and getTwissTransform() to all elements.