Added functionality for computing the crossing number of digraphs, both actual values and upper and lower bounds.

Also added miscellaneous functionality to support this goal:

• Generation of cubic and semicomplete digraphs
• Checking if a digraph is cubic or semicomplete
• Adding of an artificial vertex between two given edges
• Computing of all three cycles and triangles in a digraph
• Computing the size of the partitions of a complete multipartite digraph (I feel this was probably achievable before but I could not find the documentation for such a method).

All functionality has been tested, with all public-facing functionality documented.