Resolve fit differences

[mikeyrf]

test_stable_fit produces a fit that differs significantly from our Mathematica fit in tests/helpers/fit.csv

stdout

E    AssertionError:
E    Not equal to tolerance rtol=1e-05, atol=0
E
E    Mismatched elements: 5 / 5 (100%)
E    Max absolute difference: 0.01988231
E    Max relative difference: 10.92599192
E     x: array([ 1.327829e+00,  8.168355e-02,  6.409443e-04,  0.000000e+00,
E           -2.527482e-05])
E     y: array([ 1.307946e+00,  8.926380e-02,  6.353052e-04, -1.100660e-04,
E           -2.119305e-06])

tests/test_pystable.py:147: AssertionError