Copy constructor and assignment operator are unnecessary

[AGFeldman]

In maze/src/common.hpp, we define a copy constructor and an assignment operator for the Coordinate class. But, these definitions are completely unnecessary, since the default copy constructor and default assignment operator do the exact same things (see http://www.cplusplus.com/articles/y8hv0pDG/). We should remove these two definitions and update the assignment writeup. This will be one fewer thing that students need to deal with.

[AGFeldman]

Thanks @sshackfo for bringing this up