v0.8.8

• Added the ability to read and write files according the HIF functionality standard #572 (@nwlandry).
• Implemented the add_node_to_edge and remove_node_from_edge methods for DiHypergraphs #571 (@nwlandry).
• Allow empty edges #565 (@nwlandry).
• Simplified the cleanup() methods #569 (@nwlandry).
• Fix Issue #566 #567 (@maximelucas).
• Added documentation about N vs. N-1 in the var() and std() methods in the stats module #562 (@nwlandry).
• Fix Issue #552 #561 (@nwlandry).