Cycle detection in commodity_graph detecting too many cycles

[idelder]

Under certain circumstances a combinatorial problem arises in cycle detection. Example network below: If I got this right (great way to kill an hour), there are 81 simple cycles through this network (doesn't visit any commodity twice)

length	combinations	example
6	36	1A2B3C'
5	0	-
4	36	1A2B'
3	0	-
2	9	1A'
total	81

Put a few of these blocks together and it quickly explodes. We had a case where so many cycles were being output to the log file that the log file size hit 56 GB, couldn't be opened, and slowed the model run to a crawl.

Low priority but possible fixes:

1. Check how many simple cycles were found. If it is very large, then just log "N cycles detected [...]". This doesn't avoid the slowdown of actually finding all those cycles though, which I haven't tested yet.
2. Have a counter check on the cycle iteration block. Log a few cycles then log like above.
3. Have a configuration option to disable just the cycle check, in case the detection itself is very slow and should be skipped but we still want orphan detection.