Code Improvements

[robertapplin]

Some improvements/changes can be made to some parts of the code in this repo:

• cpp The copy constructor and equals assignment operator should probably be deleted in the NBodySimulator because it has a member variable which is a vector of unique ptrs.
• cpp const & on some of the return types can be used.
• Primitive data types passed as parameters to functions can be const.
• Pass by Value in Constructors as explained in Pass Parameters by value for Constructor #113
• Convert cpp code to use C++ 20 . Some things to improve include:
  - Using std::erase_if instead of std::remove_if and std::erase