Expression interface

[glebbelov]

For solvers accepting expression trees (SCIP 9, Xpress, Lindo), extend MP FlatConverter to support them.

Advantage: better numerical properties (see SCIP 8 release paper.)

Scope:

• Allow mixing flat constraints with expressions. For example, SCIP supports resvar = AND(v1, ..., vn), along with expressions.
• Keep the existing automatic redefinition mechanism.
• Allow per-expression type setting, whether it should be passed to the solver, or redefined - similar to flat constraints.
• This should obvously coexist with the mp::NLHandler and mp::Expr interfaces passing expression trees directly from the NL file. We need clear documentation when to use which.
• Inline expressions wherever reasonable, e.g., linear expressions into each other.