plot whole ODE steps
let p4estindent work
rename evaluation of update x from velocity to euler
let the euler function tell update_x if the particle has left the world
ADD heuns method
ADD Runge Kutta method to solve (x,y)' = (-y,x)
ADD analytic solution for (x,y)' = (-y,x) for comparison
test Runge Kutta for a longer period with one particle