SOS redefinition

[glebbelov]

SOS2 redefinition currently works for SOS2 produced from MP's own PL expressions only. Thus, cvt:sos2, which receives SOS2 from AMPL's linearized PLs, is disabled by default. Only AMPL's arithmetic linearization of PLs is passed to the solver.

• With cvt:sos2=1, the arithmetic lnearization should be deleted.
• With cvt:sos2=1 should restore (globalize) the original PL and pass it to the solver if supported.
• Could add a generic SOS2 redefinition.
• SOS1 should be checked as well.