Refactored the vector class.

 - Added a normalization function to normalize vectors.
 - Changed the point type to doubles to allow for accurate normalization.
 - Separated vector.hpp into two files: vector.h and vector.cpp since the normalization function couldn't be in the header.
 - Updated all files which include vector.hpp, they now include vector.h.
 - Changed a cast in src/GameStates/SinglePlayerGameState.cpp to a double to avoid the narrowing conversion warning.