<summary>Linear programming and integer programming</summary>

See the comments at the top of linear_solver.h for an introduction to
linear programming and integer programming.

- linear_solver.h: the point of entry for the wrapper that provides a
  simple and unified interface to several linear programming and mixed
  integer programming solvers.

- linear_solver.cc: the C++ code of the linear solver wrapper that is
  common to all solvers accessible through the wrapper.

- *_interface.cc: each file corresponds to one of the solver accessible
   through the wrapper.

- linear_solver_test.cc: the C++ tests for the wrapper.

- python/... : the SWIG code that makes the wrapper available in Python,
  and its unit tests.

- java/... : the SWIG code that makes the wrapper available in Java,
  and its unit tests.

- csharp/... : the SWIG code that makes the wrapper available in C#,
  and its unit tests.

- linear_solver.proto: the protocol buffer for mathematical
  programming problems.