ute the variance along given axis while ignoring NaNs

    Parameters
    ----------
    values : ndarray
    axis : int, optional
    skipna : bool, default True
    ddof : int, default 1
        Delta Degrees of Freedom. The divisor used in calculations is N - ddof,
        where N represents the number of elements.
    mask : ndarray[bool], optional
        nan-mask if known

    Returns
    -------
    result : float
        Unless input is a float array, in which case use the same
        precision as the input array.

    Examples
    --------
    >>> from pandas.core import nanops
    >>> s = pd.Series([1, np.nan, 2, 3])
    >>> nanops.nanvar(s.values)
    1.0
    r