Added generator types

We should learn from other games and add some new generator types, such as bio-generators, fuel generators, coal generators, hydrogen generators, heavy oil generators, wind generators, and ocean generators, to make the game more replayable.

It might be good if you explain a bit more in detail what you'd like to see, as most of these are currently in-game.
Generators run on mechanical power. Mechanical power is produced via steam
Bio-Generators: Biomass can be burnt to produce steam
Fuel Generators: Various fuels can be burnt to produce steam (includes heavy oil and hydrogen)
Coal Generators: Coal can be burnt to produce steam
Hydrogen generators: Fuel Cells might be interesting, especially as you could use hydrogen as power storage.
Wind generators: I'd like to see that
Ocean-based generators: More incentive to not put landfill everywhere, sounds good.
Keep in mind that the reason diesel generators don't use steam is because it is used in a combustion engine, instead of using the heat as would be the case with coal, heavy oil, biomass or other burnable products. Making a generator that runs directly on coal for example makes no sense in a realistic approach.