Discontinuity in q-space paths

Per Paul Neves, it would be nice to adapt `q_space_path` and `plot_intensities` to allow for discontinuities as in the image below: <img width="720" height="294" alt="Image" src="https://github.com/user-attachments/assets/6bbd1067-74a1-4f53-9329-d59bbc245785" /> Syntactically, this might be achieved with a pair of characters "/" around the labels to mark the discontinuity. E.g. ```jl q_space_path(cryst, [q1, q2, q3, q4]; labels=["L", "K/", "/U", "X"]) ```