Gordian Edenhofer authored Jan 23, 2020 and Martin Reinecke committed Jan 23, 2020

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.

Make the ScalingOperator consistent with `from_global_data`, `full` and
possibly other operators which take the domain as first argument. This
breaks the currently enforced consistency with the `DiagonalOperator`
which takes a field as first argument and the domain only as an optional
keyword argument.

Fixes #279.

Amend the operator section as well to make both functions also work on
chains of operators.

Reviewed docs on operators, library, sugar. Fixed bug in makeOp due to the recent BlochdiagonalOperator interface change

Multiplication of operators had worked before. Only multiplication with scalars
was not possible.