Implement "copy" functionality for FGraph

**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

Start by locating the FGraph definition and the existing mutation operations in the F# source, then review the MemberwiseClone reference linked in the issue. Done means providing a deep-copy operation whose returned graph can be changed without affecting the original; add or run focused tests for that independence.

Written by the indexing model from the issue text.