IGraph/M

Changes to exsiting functions:

• IGReverseGraph now supports non-simple graphs.
• IGLuneBetaSkeleton is now interruptible for β < 1.

Fixes:

• Several functions failed to check if the graph was mixed (having both directed and undirected edges). This is now corrected.
• IGLayoutBipartite now uses partitions which are consistent with IGBipartitePartitions.

Other changes:

• Many corrections and improvements to the documentation.