Release 0.4.0

[ben9583]

• Added a trail quality setting that lets users set the number of particles drawn to the screen
• Added a simulation accuracy setting that lets users set the accuracy of the simulation by choosing whether to use more or less simulation ticks per time step
• Improve the performance of the simulation several times over through a complete refactor of the code
• Set the default accuracy of the simulation to run 5 ticks per time step (instead of 1). This corresponds to 'medium' on the new simulation accuracy. The previous accuracy can be found at the 'low' setting
• Improve the accuracy of the Lagrange point demonstration by using higher precision calculations

Closes #6.