Some basic refactoring in SpaceObject class.
There's no need to teleport a space object.
Moved the test for RungeKutta4 from RosettaCode to the class where all tests for RungeKutta4 should be.
Changed the name of the variable in respect to which we derivate to x from t.
Changed the order of operations in the returning line of RungeKutta4Vector to make it more clear.
Changed the SpaceObject class to always default with a zero vector for the velocity. Added tests for updating methods as well.
Redesigned Gravity class (probably fixed the formula as well).
Fixed the possible division by zero in Gravity class.
Small changes in Update class.