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.

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