Fix the __eq__ method of GeneralGraph and add __hash__

[Ykabrit]

This PR resolves #251 by modifying the __eq__ method in GeneralGraph.
As implemented initially, this method breaks graphs by sorting self.nodes in-place (which renders the list of nodes incoherent with self.graph and self.node_map), as well as returning False when two identical (but having different node ordering in self.nodes) graphs because it compares self.graph and other.graph which are not identical if the node ordering was different.

In the modified method, self.nodes and other.nodes are both sorted using the sorted() method and stored in local variables. Secondly, rows and columns of other.graph are permutated in order to correspond to self.graph and make them comparable, which was not the case before.

Not entirely related, but I also added __hash__ methods to GeneralGraph and Edge.

[zhi-yi-huang]

Thanks for your contribution! I have one suggestion regarding the hash calculation: instead of computing hash(node_tuple) + hash(graph_tuple) and using that sum as the combined hash, it would be better to combine the two tuples into a single tuple first and then hash that—i.e., hash((node_tuple, graph_tuple)). This helps to better maintain the uniformity and uniqueness of hashes.

[Ykabrit]

You're right! Here, normally it's fixed.

[zhi-yi-huang]

Thanks!