Made the return value of velocity() const to check bugs.
Made the return value of velocity() const to check bugs.

Prior to this, I often accidentally wrote obj->velocity() += whatever.
   which compiles with no errors or warnings, but is always a bug.
   This change makes it an error.

elidupree/Lasercake,Lasercake/Lasercake,Lasercake/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,idupree/Lasercake,elidupree/Lasercake,elidupree/Lasercake,elidupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake