Use a unit root function to determine the minimum number of differences necessary to obtain a stationary time series.

```
unitroot_ndiffs(
x,
alpha = 0.05,
unitroot_fn = ~unitroot_kpss(.)["kpss_pvalue"],
differences = 0:2,
...
)
unitroot_nsdiffs(
x,
alpha = 0.05,
unitroot_fn = ~feat_stl(., .period)[2] < 0.64,
differences = 0:2,
.period = 1,
...
)
```

- x
A vector to be tested for the unit root.

- alpha
The level of the test.

- unitroot_fn
A function (or lambda) that provides a p-value for a unit root test.

- differences
The possible differences to consider.

- ...
Additional arguments passed to the

`unitroot_fn`

function- .period
The period of the seasonality.

A numeric corresponding to the minimum required differences for stationarity.

Note that the default 'unit root function' for `unitroot_nsdiffs()`

is based
on the seasonal strength of an STL decomposition. This is not a test for the
presence of a seasonal unit root, but generally works reasonably well in
identifying the presence of seasonality and the need for a seasonal
difference.