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