[Help-glpk] OPB-output patches

[Oscar Gustafsson]

Dear Andrew,

please find included fixes for the following two issues when exporting a 
problem as an OPB-file:

* Constant objective function terms were ignored
* Empty LHS were ignored

Also, I've included an experimental version of binarizing integer 
variables, which work with the normalized output format. Invoked by the 
"--binarize" switch.

With best regards

Oscar Gustafsson

diff-4.28
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