test quadratic MPS writer

[mlubin]

This fixes an apparent bug that variables which had 0 linear objective coefficients didn't appear in the list of columns, which Gurobi didn't like. It also seems that we don't need to duplicate the off-diagonal terms in the QMATRIX section (@cmaes?).

[mlubin]

Cplex seems to say that you do need to provide the lower and upper triangle entry for off-diagonal terms, but gurobi doesn't. Should we care?

[cmaes]

From http://www.gurobi.com/documentation/6.0/refman/mps_format.html

The next section in an MPS file is the optional QUADOBJ section, which contains quadratic objective terms. Each line in this section represents a single non-zero value in the lower triangle of the Q matrix. The names of the two variable that participate in the quadratic term are found first (starting in columns 5 and 15 in fixed format), followed by the numerical value of the coefficient (in column 25 in fixed format). By convention, the Q matrix has an implicit one-half multiplier associated with it. Here's an example containing three quadratic terms: