UF: Adding the traveling salesman problem (TSP) linear programming version

[alejomonbar]

Adding the traveling salesman problem class with its corresponding test.
https://en.wikipedia.org/wiki/Travelling_salesman_problem

Description

Adding the traveling salesman problem combinatorial optimization problem to the set of problems. This problem comes with the function 'classical_optimization' which uses the 'docplex' solver, plot_solution which plots the 'networkx' graph for a given solution, and a get_distance function to get the total distance traveled from a given solution.

Additionally, I added a subfolder to organize the tests for the problems.

Checklist

• I have performed a self-review of my code.
• I have commented my code and used numpy-style docstrings
• I have made corresponding updates to the documentation.
• My changes generate no new warnings
• I have added/updated tests to make sure bugfix/feature works.
• New and existing unit tests pass locally with my changes.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

the test is inside './tests/test_problems/test_tsp.py'