Allow thermogenerator to use world temperature instead of lava

Hello,

I thinked about a way to generate electricity from world temperature. But there was already a thermogenerator. So I think, why not allow it to use world temperature instead of lava (while keeping water consumption), but with a lesser energy generation ?

This is what this PR do. If no lava source connected, it will start to generate 0.025 energy per degree (to be multiplied by 3 if fully immerged). This require a world with stricly more than 0 degree as temperature.

Let say the world is at 10 degrees and the thermogenerator fully immerged. It will generate 0.75 energy, while connecting it to lava would generate 6 energy. So we need 8 thermogenerator without lava to do the same than 1 thermogenerator with lava (if the world is at 10 degree).

This add a new funny (because it require water ^^) way to generate energy, and it also work in night (but temperature decrease during night !).