Paint red balls
Fix Vector.get_direction
Add vector.__abs__
Add vector.__radd__
Fix vector.__add__
Compute total energy of a body
Remove body.draw
Forces acting on a body are no longer hold in list
PhysicalBody no longer needs apply method
Remove body.size
Replace numerous updates with just one
Replace String.apply_force with String.apply
Strings apply potential energies to bodies
Update grid_generator
Remove Integrator.keyPressEvent
Remove Integrator.paintEvent
Integrator no longer extends QWidget
Add normalizing functions
Add energy to Integrator
Add Integrator.get_total_energy