20091104
  gurobi.c, README.gurobi: fix a glitch with iisfind=1 on infeasible
problems:  solve_result_num previously indicated a failure rather
than infeasible.

20091214
  gurobi.c: add "cutoff" keyword to possibilities for $gurobi_options.
  makefile.u: add rule for xsum.out.
  README.gurobi: add brief MacOSX discussion.

20100309
  gurobi.c: fix bugs in handling .ref and .sosno suffixes.

20100418
  gurobi.c:  extend for Gubobi 3.0; should continue to work with older
versions of Gurobi as well.  New facilities include keywords for
requesting .sol files of alternative MIP solutions; new synonym
lpmethod for simplex and a way to select the (new) barrier algorithm
(lpmethod=2); new tolerances and an iteration limit for the barrier
method; control of crossover from barrier solution to basic solution;
"mipfocus" to help direct the MIP solver; "predual" to present the
dual of an LP to the solver; "pumppasses" to control the feasibility
pump; and "symmetry" to control the MIP solver's detection of
symmetry.  See the output of "gurobi -=" for a summary of the
currently available keywords.  Complete list of new keywords for
Gurobi 3.0:  aggfill, ams_eps, ams_epsabs, ams_limit, ams_stub,
barconvtol, barcorrectors, bariterlim, barorder, crossover,
crossoverbasis, lpmethod, mipfocus, mipgapabs, networkcuts, param,
paramfile, predeprow, predual, prepases, pumppasses, resultfile,
rins, submipcuts, symmetry.

20100424
  gurobi.c:  fix a couple of typos in the "-=" output; correct
spelling of "prepasses" and for now make "prepases" a deprecated
synonym for "prepasses".

20100512
  gurobi.c: adjust for a gratuitous parameter name change
("LogfileName" to "LogFile") between versions 2 and 3.

20100523
  gurobi.c: adding keyword return_mipgap for returning suffixes
absmipgap and relmipgap on objectives and problems.  (These values
could already be computed from the bestbound suffix.)

20100720
  gurobi.c: distinguish explicitly zero initial guesses from those
that are zero by default.  This affects MIP starting guesses.

20100928
  gurobi.c: expand description of sos2 (shown with "gurobi -=").

20101021
   gurobi.c: fix some bugs in statmsg() with error messages not
normally seen.

20101105
  gurobi.c, README.gurobi: update for Gurobi 4.0.

20101115
  gurobi.c:  fix a bug (introduced in adjustments for Gurobi 4) that
sometimes caused the constant term in objectives to be treated as 0.
This affected the objective value reported.

20101207
  gurobi.c, README.gurobi:  set solve_result_num = 567 if the problem has
complementarity constraints.

20110322
  gurobi.c: fix a bug in handling problems with integer variables
(including binary variables) and piecewise-linear terms:  the wrong
variables were likely treated as integer.  Fix a bug with "writeprob"
that could occur in problems with piecewise-linear terms:  a call on
GRBupdatemodel() was missing, which could affect, e.g., a *.lp file
specified by writeprob.

20110426
  gurobi.c, README.gurobi: update to Gurobi 4.5.  The updated gurobi.c
should still work with earlier versions.  The new .unbdd and .dunbdd
suffixes (for primal and dual rays of unboundedness) are not yet
reliable.
