Derived datasets: Remove dummy scaling attributes from DerivedDataset class

[ploh]

Currently, the scaling attribute is defined with

attribute :scaling,      Preset::Scaling

in DerivedDataset. But since the Preset::Scaling has attributes has_energy, has_agriculture and has_industry which are not respected when set for a DerivedDataset#scaling, this is not ideal - also, it is unnecessary, since the DerivedDataset has all the attributes of a Dataset directly, among them a lot of has_... attributes (has_energy is missing in the attributes of Dataset. Maybe we should add that? @antw @ChaelKruip).

Solution:
The Preset::Scaling class should get a newly invented superclass Atlas::Scaling without the has_.... This superclass is then used for the DerivedDataset#scaling.

[ChaelKruip]

@ploh why am I assigned for this ticket? I have no idea what to do with this...

Re-assigning @ploh

[ploh]

@ChaelKruip Yes, sorry, I should have commented why I did that. This is, of course, not for you to implement, but for @grdw - who is also assigned. I assigned it to you as well because I thought you are probably the one to decide / prioritize when/if Gerard should do this.