Power plants that produce goods

[poppo]

I made the patch which allow power plants to produce goods.
electricity_and_goods_producer.patch
We can already make the goods-producing power plant pakfile which name is including "power plant" or "kraftwerk". But current version, these factories does not produce electricity. So, I add fabrik_t::set_power_supply() for all cases in fabrik_t::step().

[prissi]

This affects the balance between current electricity and overall electricity for power plant generation, if power plants are not end users.

I think this needs a little more changes to become balanced. (The eternal construction site of the factory creator, which probably need really badly an overhaul.)

[poppo]

I know power plants capable of generating electricity in this way can affect the game balance. But I think it is necessary to run them that can be created as .pak files and are already recognized by the system as producing power plants (we can already build pumpe_t with them in current version but forget to call set_power_supply()).

I want to determine the powe_supply values for them to become balanced.