The integrator is in charge of updating the world state with the current
bodies and forces in action.
author  jeffrey traer bernstein (original source code)
author  Johann Sorel (adapted to unlicense project)

Get the collision solver used to update the physique world.
return  WorldSolver

Set the collision solver used to update the physique world.
param  solver

Update the given world.
param  world
param  timeellapsed : in seconds if using conventional units