This is best practise in python scripts in order no to clutter the global
namespace.

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.

Although it is working

This is a performance tweak. Since inverse diagonal operators are automatically
converted to a pure diagonal operator with the inverse on its diagonal, this
commit saves memory.

Furthermore, the zero_mode keyword is gone for AmplitudeOperator. It is not used
in the demos and is not really appropriate from a theoretical point of view
anyways. We will add in a better option later.