forward fill missing values

def ffill(arr, dim=None, limit=None):
    '''forward fill missing values'''
    import bottleneck as bn

    axis = arr.get_axis_num(dim)

    # work around for bottleneck 178
    _limit = limit if limit is not None else arr.shape[axis]

    return apply_ufunc(bn.push, arr,
                       dask='parallelized',
                       keep_attrs=True,
                       output_dtypes=[arr.dtype],
                       kwargs=dict(n=_limit, axis=axis)).transpose(*arr.dims)