groupNames: Default is NULL which plots all the microbes. To plot a subset of all the groups, specify a vector of strings of the names of the groups you want to plot.
sumOverPaths: Logical. Default is TRUE which sums flows between the same nodes even if they are on different metabolic paths
sumOverStrains: Logical. Default is TRUE which means the strains are put into their functional group nodes and the flow are summed. When it is FALSE, each strain will have its own node.