Closed requested to merge more_restrictive_scaling_operator into NIFTy_6
ScalingOperators do not know the dtype, on which theyu operate. This can lead to problems if complex scalingOperators act on real fields, as this causes it to cast the input to a complex number. However, the adjoint then does not cast this number back, leading to inconsistencies. This branch therefore raises an error whenever a ScalingOperator with complex factor acts on a real input, forcing the User to use an adjoint Realizer to cast the field to a complex one first. The DiagonalOperator should probably do the same check.