Make use of the compaction report to create a variable dependency graph

[nickerso]

771cde2 starts to add better functionality to the model compaction report to track how variables are requested to be compacted and how they are resolved, what dependencies get triggered, etc. Currently this information is just used to try and help identify errors in the source model (the original reason for creating this tool). But there is probably sufficient detail now being stored in the report class that we could look at generating dot files for use in graphviz (i.e., similar to CellML2Dot) or some other network visualisation software.

This sort of feature might help debug models where the compaction process itself doesn't produce any errors...maybe?