Backpropagation of Twiss parameters, bugfixes
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.