Numerical instability in the derivative of the upper gamma function

[AlexisFremond]

The computation of the derivative of the upper gamma function in

def _jac_uppergamma_c(c: np.ndarray, x: np.ndarray) -> np.ndarray:
     return shifted_laguerre(lambda s: np.log(s) * s ** (c - 1), x, ndim=np.ndim(x))

is unstable for low values of the parameters. The numerical result diverges from the true value as parameters go to 0. Test example : c = 0.001, x = 0.002.

[21ch216]

This issue will be treated in the next weeks