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:

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.

This reverts commit aa3cdd2c.