This container holds all the state of bodies in an island. Dynamics will only appear in one island, but kinematics are in multiple and so their state will be duplicated in each island. This is ok - we do not write to the state of any kinematic bodies in the constraint solvers. We also assume that the number of kinematics is small compared to the number of dynamics. If this is commonly untrue we may want to consider having a separate (global) container of kinematic solver bodies.