Internalise null model generation

[miratepuffin]

Python Raphtory has a small module for null models which takes a dataframe and shuffles the data either by time or on destination - returning a raphtory graph.

This allows a user to run an algorithm on their real data and then compare this to a very similar graph to see if their results are significant.

An improvement to this would be to bring this functionality into the graph gen module in rust - this would allow users to call the desired null model on their graph and be returned with a new shuffled graph - thus cutting out the need for the dataframe.

[DanielLacina]

I'll try this.