Docs Tutorial: Python API invalid sm_min and sm_max import

The imports `sm_min` and `sm_max` are no longer a part of the `stats` module.

From line `12` of the Python API Usage tutorial:
```from silvimetric.resources.metrics.stats import sm_min, sm_max, mean```

### Possible Solution

It looks like `min` and `max` require a defined Metric, similar to what `mean` has in `p_moments.py`:
```mean = Metric(name='mean', dtype=np.float32, method=m_mean)```

