ENH:stats: Add _isf method to lomax

[OmarManzoor]

Reference issue

Towards: gh-17832

What does this implement/fix?

• Adds the _isf method to the Lomax distribution in order to improve its precision.

Additional information

[OmarManzoor]

from scipy.stats import lomax
import numpy as np
import matplotlib.pyplot as plt

def _isf(q, c):
    return q**(-1.0/c) - 1.0


def plot_isf():
    c = 2.0
    q = np.logspace(-40, -1, 1000)
    plt.semilogx(q, lomax.isf(q, c), label="main", ls="dashed")
    plt.semilogx(q, _isf(q, c), label="pr", ls="dotted")
    plt.legend()
    plt.title("lomax inverse survival function")
    plt.show()

[mdhaber]

This looks like it will be in good shape if it passes the new test.

[mdhaber]

But you need to add it to the list. That test is opt-in because so many distributions didn't pass it - even the ones that override _sf and _isf