[stats] fix formulas for higher moments of R, Burr, Fisk and F distributions

Also rework the stats method of F to avoid having spurious division-by-zero warnings.
The latter were generated because of the dispatch on numpy.where: 
```
>>> a = np.array([1, 2, 3])
>>> np.where(a > 1, 1./a, np.nan)
```
(caught by Warren in gh-2849). 

There is still a broadcasting failure left: `f` has two shape arguments, and `f.stats` fails when called with different lengths: `f.stats(11, [1, 2, 3])`.
I'm pretty sure this is not the only distribution which does this, thus am marking this test as a knownfailure. Until we fix it generically.