Add unit tests

[jialuw96]

Add unit tests, including:

• test kinetics examples with A- and D-optimality, mixed-integer or relaxed in Pyomo
• test kientics examples with A-optimality MILP, and D-optimality NLP in Cvxpy
• test functions, including MeasurementData, SensitivityData, and all five types of covariance matrix

[jialuw96]

@adowling2 Please see the following for the unit tests added. The instructions on what are them and how to run them are also updated in the readme file. Thank you!

• test_example.py: test the kinetics example in Pyomo in A- and D-optimality, mixed integer and relaxed
• test_cvxpy.py: test the kinetics example in Cvxpy in A-optimality MILP and D-optimality NLP problems
• test_function.py: test the SensitivityData, MeasurementData objects, and all five types of covariance matrix

[ksbeattie]

@adowling2 what about this one?