Summary report for external plugins

Some external plugins, such as the PP plugin, do not expose a PwCalculation, which makes it difficult to produce a unified summary.
One solution is for each plugin to include its own summary section. The main summary page can then gather these sections together. Similarly, the relaxation step (treated like a plugin) would have its own summary. This way, if only the PP plugin is run, no relaxation summary will be displayed

@edan-bainglass , @superstar54