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Adapt the ScalingOperator class to not depend on sugar.py in order to
use it within sugar.py without getting a circular dependency.

reverted changes in linearOperator, put changes in scalingoperator instead, fixed bugs and wrote tests:

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.