added tf-based enrichment + other minor changes to the commiunity enrichment and display
- added a function to calculate TF enrichment (based on either degree centrality, eigenvector centrality, or a custom set of TFs)
- added a function to plot the TF enrichment
- added a function to retrieve top nodes
- if the user tries to plot GO enrichment results for a TF or community for which the results haven't been previously calculated, they are automatically calculated in the plot function (with the default parameters).
- Enrichment is not calculated for all communities by default