Adding new tests to increase coverage

I don't think this is quite right.  With this setup, if someone specifies only features to filter, but also sets `inverse` to `True`, they'll end up with nothing in `self.ids` and `self.classes`, but have items left in `self.features`.  

The `FeatureSet` class requires that `len(self.ids) == len(self.classes) == len(self.features)`.  

Anyway, what I think we want to do instead is change

```python
if inverse:
    mask = np.logical_not(mask)
```

to

```python
if inverse and (classes is not None or ids is not None):
    mask = np.logical_not(mask)
```