Use Vector3D in getField method (was ACTS-274)

Original author Julia Hrdinka @jhrdinka

Currently the getField method in the RungeKuttaPropagator uses an array of double as an input parameter for the position and an array of double as output parameter for the magnetic field at the given position. Both, position and BField should be described as a Vector3D using our underlying math library.

{code:java} // returns the 3D BField at position Vector3D getField(const Vector3D position) const { return BFieldVector; } {code}