add basic engine class with game loop and mechanisms for handleing entities and systems
renames several functions in namespace ecs and adds const do some member functions
removes two wrong const from member functions
adds defaulted copy assignment and move assignment operator to basic engine
adds missing semicolon in basic engine
adds pointer to engine object to basic system so that other systems can be accessed
adds engine example to main
removes noexcept from defaulted copy constructor and move/copy assignment operators. fixes compilation error caused by wrong functon name