Multi-edges

[moaxcp]

Currently graphs are strict. Only one edge can connect ant two vertices. There is no type for multi edge graphs which allow multiple edges between vertices (except directed).

In graphviz graphs are multi edge unless they are defined with the strict keyword.

#task Add multi-edge types Make multi-edge the default behavior 'graph' and 'directed-graph' will be multi-edge types 'strict-graph' and 'strict-directed-graph' will be strict