Xpress: sos constraint do not support linear expression

like gurobi, xpress solver do not support natively linear sos contraint... But contrary to gurobi_solver.cc which introduce temp variable to make it work out of the box, xpress did not

ref:

• https://www.fico.com/fico-xpress-optimization/docs/latest/solver/optimizer/HTML/xprscxx/classxpress_1_1objects_1_1SOS.html

• https://www.fico.com/fico-xpress-optimization/docs/latest/mosel/mosel_lang/dhtml/makesos1.html

• https://docs.gurobi.com/projects/optimizer/en/current/reference/c/model.html#c.GRBaddsos