remove all old demos, GN prototype in

demos/critical_filtering.py

demos/getting_started_1.py

demos/nonlinear_critical_filter.py

demos/nonlinear_wiener_filter.py

demos/paper_demos/cartesian_wiener_filter.py

demos/paper_demos/wiener_filter.py

demos/poisson_demo.py

demos/probing.py

demos/sampling.py

demos/tomography.py

demos/wiener_filter_data_space_noiseless.py

demos/wiener_filter_easy.py

demos/wiener_filter_via_curvature.py

demos/wiener_filter_via_hamiltonian.py