OR-Tools: use MathOpt instead of MPSolver

[pet-mit]

Describe the current behavior

OpenRAO uses the "MPSolver" object in the OR-Tools library as an interface with the solvers.

Describe the expected behavior

OpenRAO must use "MathOpt" instead of "MPSolver" in the future.

Describe the motivation

Google is dropping support of the MPSolver interface: google/or-tools#3992

Extra Information

Two pre-requisites:

• MathOpt should have a Java API (Next major release will be 10.0 google/or-tools#3992 (reply in thread))
• MathOpt should have XPRESS support (Add XPRESS support to MathOpt rte-france/or-tools#130)