Define the structure `ThreeDvector` that represents a 3 dimensional vector in (for convention in physics the vector are represented as ai + bj + ck where a, b, and c are real numbers and i, j and k represent the direction x,y and z respectively in the Cartesian plane there for we use the names i, j and k for the fields in the ThreeDVector structure
Look how the operations Addition and Subtraction work for a 3 dimensional vector and implement them by implementing the std::ops::Add and std::ops::Sub traits