Version3_1, example_08: corrected alpha shift by 180

[rapoliveira]

The commits change the Python script and jupyter notebook for example_08, regarding the change of alpha by 180 deg. The results presented slight variations, for which the expected outputs were updated.

In one of the last commits, I suggest to add pytest.warns to handle/catch a warning related to mag or flux residuals. Note that it doesn't ignore the warning, but catches it inside the test without showing yellow in the terminal.

@rpoleski the results change in example_16 only for ob03235_1 and ob03235_2_full, which include alpha. I wasn't able to make the required changes and leave it to you. The inputs ob08092-o4_prior_1.yaml and _2 generate the following error, which will probably be solved after releasing Version3.

  File "/Users/raphael/postdoc/MulensModel_raphael/examples/example_16/ulens_model_fit.py", line 2678, in _extract_posterior_samples_EMCEE
    for parameter in self._parse_posterior_abs:
                     ^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'UlensModelFit' object has no attribute '_parse_posterior_abs'

[rpoleski]

Please move discussion of ex16 to a separate thread. Thanks for all the changes.