Fix multigraph color change

[boldar99]

The color_change function had been broken with the Multigraph implementation.

Changes:

• The implementation of color_change has been modified to call a new function that toggles between Hadamard and Simple edge types.

• The toggle_edge utility function has been changed not to do anything when the input edge type is neither Hadamard nor Simple.

• Implement __repr__ for multigraph.Edge for better error messages

Related to zxcalc/zxlive#256

[jvdwetering]

I'm not sure if we should maybe play it safe, and raise an exception if toggle_edge is called on an edge that we don't expect to be toggleable, instead of just silently doing nothing.

[boldar99]

That's a good point! I just pushed a fix.

[RazinShaikh]

I am not sure if toggling the edge type for multigraph should simply flip the number of simple and Hadamard edges. It makes sense for colour change but what if in a different context, switching the edge type of only the chosen edge (not all edges) makes sense?

[RazinShaikh]

superseded by #244