Implement "copy" functionality for Graph and DiGraph #53

Description
Our ultimate goal is to leave the decision to use mutation or not to the user. Mutation makes sense for heavy duty operations that run on the cloud. But while developing and working with smaller graphs mutation gets in the way and can be unwieldy to use. The initial step for this is to have a deep copy functionality.

Pointers
https://learn.microsoft.com/en-us/dotnet/api/system.object.memberwiseclone?view=net-7.0