numpy.std filed in element wise groupby + apply
From [SO](https://stackoverflow.com/q/46278161/2901002):

    np.random.seed([3,14159])
    df = pd.DataFrame(
        np.random.randint(10, size=(3, 3, 5)).tolist(),
        list('XYZ'), list('ABC')
    ).applymap(np.array)
    
    df.loc['X', 'B'] = np.nan
    df.loc['Z', 'A'] = np.nan
    print (df)
    
    df2               = df.stack().groupby(level=1)
    element_wise_mean = df2.apply(np.mean, axis=0)
    print (element_wise_mean)
    A                        [4.0, 5.5, 2.5, 1.0, 7.0]
    B                        [5.0, 2.5, 7.0, 4.5, 7.0]
    C    [5.66666666667, 3.0, 4.0, 4.0, 8.66666666667]
    
    element_wise_sd   = df2.apply(np.std, axis=0)
    print (element_wise_sd)
    
>TypeError: setting an array element with a sequence.

I try find solution and if [cast to numpy array](https://stackoverflow.com/a/46278283/2901002) it working nice.

I know pandas not support arrays using this way very nice, but maybe it is simple bug.

Thanks.

