Add support for a sponge radius based on the distance to the nearest element to the specified boundary nodes (rather than a fixed radius away from the boundary). To enable, set the SpongeRadius value given to add_sponge_nodes_list.m to a negative value and then call calc_sponge_radius.m to assign sponge layer coefficients to the relevant nodes. Finally, call add_spong_nodes_list to add the nodes to the MATLAB mesh struct