Added function `moment`

This is my first contributed function (i.e. please someone check thoroughly). I've checked the basic functionality against circstat (matlab), and added a number of tests. 

Questions and issues:

1. Currently the arguments `w` and `d` only alter the behaviour of this function if `cent=True`. Should they be doing something additionally in the body of the function?

1. I return the values in a tuple. I notice that most of our other functions return only single values, making them more compatible with e.g. the bootstrap decorator.

1. Further to this: does it make sense to include bootstrapping for the moments, and if so, how should this be done (a simple attempt broke because this function outputs a tuple).

Assuming this function is acceptable, I plan to do skewness and kurtosis over the weekend.
Ah, of course. Sorry.

I will fix it today.
On 11 Nov 2014 07:47, "Fabian Sinz" <notifications@github.com> wrote:

> In pycircstat/descriptive.py:
>
> > +    :return:    Tuple containing three values:
> > +                mp      the complex p-th moment
> > +                rho_p   magnitude of the p-th moment
> > +                mu_p    angle of the p-th moment
> > +
> > +    Example:
> > +
> > +        import numpy as np
> > +        import pycircstat as circ
> > +        data = 2*np.pi*np.random.rand(10)
> > +        mp, rho_p, mu_p = circ.moment(data)
> > +
> > +    References: [Fisher1995]_ p. 33/34
> > +    """
> > +
> > +    if axis is None:
>
> I know that no other function uses it. I wrote it later and didn't convert
> the other functions. The trick of the decorator is that it reshapes the
> input array depending on what axis argument is given such that the desired
> axis is at axis=0. Then it also sets the axis to be 0. If you try with a 2d
> array and axis=1, the array should be transposed and axis should be 0.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/circstat/pycircstat/pull/33/files#r20134242>.
>