Failing tests

[ausonandres]

Running the tests for PR #44, it has been found that there are some of them that fail depending on where they are run. Then, this issue is meant to collect all the tests that suffer this problem.

• Every time Travis tests are run with MACOS, test_fit_slices_2d_gauss_fixed_example (in krcal.core.fit_functions_test.py) fails, however running this same test locally, or with Travis' Linux there are not any problems.

• Running the tests in neutrinos with a MACOS machine, test_fit_yields_expected_mean_std_and_chi2 (krcal.NB_utils.fit_energy_functions_test.py) also fails.

@mmkekic & @bpalmeiro will post as well the tests that fail in their cases.