Genextreme fit when c0

[swallan]

Reference issue

What does this implement/fix?

When c=0, genextreme is equal to gumbel_r. This can be leveraged in the fit method so that when c is fixed to 0, return the results of the much faster fit method from gumbel_r.:

In [23]: import numpy as np

In [24]: data = stats.genextreme.rvs(c=0,loc=2, scale=1, size=100, random_state=np.random.default_rng(432423))

In [25]: %timeit stats.genextreme.fit(data, fc=0)
673 µs ± 62.6 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [27]: %timeit super(type(stats.genextreme), stats.genextreme).fit(data, fc=0)
21.6 ms ± 1.61 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

Additional information