[Scip] explanation of use of primal heuristics

[Victor Miller]

I have a number of IP problems which I'm using SCIP to solve.  I've noticed
that in these problems that the various diving heuristics *never* find a
solution.  So I started experimenting with disabling them, and the running
time went up!  I have one small case which takes Scip 2.97 seconds to solve
on my workstation.  I noticed that coefdiving took 0.11 seconds but found
no solutions, so I disabled it.  Much to my surprise that caused the
solution time to double!  By disabling it, it allowed other fruitless
heuristics to run such as pscostdiving. But when I disabled those, others
now came into play.  I'm not completely clear on how heuristics are chosen
to be run.  I would think that it's first by priority, but what happens if
a heuristic reports that it didn't find a solution?  So far, it's been like
whack-a-mole.  I disable a bunch of heuristics which don't find solutions,
and another new one pops up.  So far, not disabling any heuristics gives
the lowest run time.
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