[Cbc] Bigger paradoxical case for extraVariables option

[acw at ascent.com]

This is a sequel to a problem I posted about back in April.

I have a 3.4 Mb MPS file that has the following behavior:

cbc -import extrav-test.mps -extrav 10 -solve -quit

takes 9723 CPU seconds to solve, while

cbc -import extrav-test.mps -solve -quit

takes 10850 CPU seconds. The difference is only about 10%, but it's in the 
wrong direction -- that is, the -extraVariables trick makes it solve 
slower. In April I presented a similar case, but Dr. Forrest pointed out 
on that occasion that the solver spent most of its time on heuristics, and 
the problem was in fact solved by heuristic; apparently introducing the 
extra variables slows down the heuristic phase.

But the new problem spends most of its time branching, and -extrav still 
slows it down, though not as dramatically as the earlier problem. I can't 
send the MPS file to the list because it's too big, but I will be happy to 
share it with anybody who is willing to take a look.
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