New GAMS solver interface, writer, and solution loader.

[AnhTran01]

Fixes NA

Summary/Motivation:

This PR introduced a new GAMS solver interface that uses the LinearRepnVisitor to enable coefficient gathering when writing out the scalar GAMS model in gms format.

Changes proposed in this PR:

• New GAMS solver interface in pyomo.contrib.solver.solvers
• New GAMS writer in pyomo.repn.plugins
• New GAMS solution loader in pyomo.contrib.solver.solvers

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