ENH: improve halfnorm CDF precision

[dschmitz89]

Reference issue

No directly related issue.

What does this implement/fix?

Increases the precision of the halfnormal distribution's CDF method in the left tail. The formula was simply changed to use the one from Wikipedia to avoid substracting 1.

Additional information

Comparison of main branch and this PR:

import numpy as np
import matplotlib.pyplot as plt
from scipy import stats
from scipy import special as sc

x = np.logspace(-17, -14, 200)
print(stats.halfnorm.cdf(x))
plt.semilogx(x, stats.halfnorm.cdf(x), label="main", ls="dashed")
plt.semilogx(x, sc.erf(x/np.sqrt(2)), label="PR", ls="dotted")
plt.legend()
plt.title("Halfnorm CDF")
plt.show()

[mdhaber]

This improves the precision at small values of the argument.