Add the possibility to provide `grad_op` to the optimization routine `stadv.optimization.lbfgs`, preventing growth of the graph. Not providing `grad_op` to this optimization routine will result in a memory leak in case of repeated calls.
0.2.0
Fix calculation of `flow_loss` to match Eq. (4) from arXiv:1801.02612. Previously the summation over p (for looping over all pixels) was done inside of the square root, and not outside.