Extract initOrder and sortLayer from order module
Add tests for initOrder
Add some tests for sortLayer
Use graph instead of tree for ordering layers
Don't store layer graphs in the main graph

Attempting to write the graph for debug purposes fails with the layer
graphs in the main graph. The error claims there is a cycle, but the
problem is not a cycle but a diamond.
Initial work on compound graph crossing reduction in one layer
Minor bug fix to initLayerGraph
Copy subgraph information from input graph to layout graph
Some more test for sortLayer
Fix log message for init order cross count
Order: only use subgraph if it has at least one node in the rank
Include subgraph information in result graph
Exclude subgraph nodes from positioning
Don't add rank to composite nodes
Ensure that composite nodes don't get an order attribute