Useful if you record multi-dimensional time-series data and want to look for periodicity. This function finds the points where the trajectory crosses a given plane.

% P = poincare_map( X [, plane] )
% given N-dimensional time-series data X,
% find points of the time series that cross a given plane.
% X ( t, variables ) is a T x N matrix of N-dimensional states evolving over time
% plane.norm = N-dim normal vector (default [1,0,0,0...]
% plane.dist = distance from origin (default 0)
% sanjay g manohar 2019