Adding visualiser

[RobertASmith]

OK so this adds some extra functionality:

• looks through functions identified by the summarise_model function and for each function identifies which functions call it.
• creates a dataframe of the edges of this network.
• uses this to identify all nodes
• produces a visual of the network.

Next steps:

• provide the information from the summary table in the network plot (i.e. user can click on a node and see the function location and any associated tests).
• wayyyy more testing - I've been dreadful at adding in tests.