Updated a few essential elements

Implemented the Velocity Object (now has a working velocity vector that
can update itself) implemented the gravity radius (can now print
aesthetic gravity field of bodies) Implemented the gravity force between
bodies (it calculates a value, but it does not affect the orbit yet)
Updated Save Module

Updated save module and got it working again, it now saves the velocity
of the body
Update to PositionCalculator

PositionCalculator is now working also added constructor to Velocity
class