Test particle in circular orbit around a solar mass particle, radius of 3AU

Compare SWIFT solution with analytic solution, and the solution from the scipy ODE solver with 10^6 timesteps per orbit.

Show the effect of decreasing 'eta'.

Now solve a similiar problem but replace the central particle with a line of 10 particles, from -0.5 AU to 0.5 AU along the x-axis, with total mass of one solar mass. Initial position and velocity of the orbiting test particle is still the same.

Can test the effect of changing both the multipole order and 'eta'

Now do the same this again but with a triangle of particles, centred on [0.5,0.5,0.5], evenly spaced around a circle of radius 0.3 AU in the x-y plane, this was done to ensure the particles were all in the same cell.