[WIP/CR] Batch crafting time reduction

We can now batch craft stuff! And it has a pleasant UI, but *batch* crafting isn't *serial* crafting, at least IRL.

The changes here represent generous and simplistic benefits of batch crafting over serial crafting. (I think the cutoff percentage should probably be higher to be not too generous...)

On the other hand making it simulation-grade would need to take into account: recipe setup time, recipe actual-crafting-time, factor for economies of scale, exact tools available, etc.

I'd like to strike a balance here making the calculation dependent only on the batch size and difficulty of the recipe, in a way that favours batch crafting but doesn't make it a cheat, hence WIP/CR status.

 
The very same value is also loaded for constructions and recipes which can get confusing and is redundant.

You should probably either
- put the `batch_time` function into the `recipe` class and use the `difficulty` that is already stored there, or
- remove the difficulty member from `recipe` and from `construction` - the `difficulty` member of class `requirements` (the new one) is than used automatically.