operator.py: Raise not-implemented in matmul call

Contrary to other overloaded operators, the `__matmul__` call apparently
needs to raise an exception instead of returning `NotImplemented`. This
behaviour can be seen for example in:

```python
psp = ift.RGSpace(512)
m = ift.MaskOperator(ift.Field.from_raw(psp, np.random.randint(0, 2, 512)))
m(ift.from_random("normal", m.domain).val) is NotImplemented
```

which should be invalid and raise an exception but previously returned
`True` instead.