Experimental batch crafting time reduction
Remove redundant get_batch_time function
Remove redundant instances of difficulty property

Difficulty is now loaded only by the requirements class, and is
automatically shared with constructions and recipes.
Logistic function batch crafting time reduction

A recipe can now have optional `batch_time_factors` array which
specifies maximum time reduction for a single craft as a percentage,
and a minimum batch size to reach the full time reduction.
Add diesel fuel to ammo.json.
Add diesel to ammo_types.json
Add diesel engines to var_vehicle_parts.json
Added diesel engines to vehicle_parts.json ( top level )
Added diesel fuel tanks to vehicle_parts.json ( top level )
Fix diesel fuel color
Added diesel to the fire code of field.cpp
Use batch_time within make_all_craft
Example batch benefits for two food recieps
Update JSON_INFO with batch_time_factors explanation
Completely omit noise and smoke from off-map vehs

I'd left the noise in, thinking it should draw hordes, but if the noise
is going to be happening inside the reality bubble and confusing
players, that's not such a great idea.
Add diesel fuel to ammo.json.
Add diesel to ammo_types.json
Add diesel engines to var_vehicle_parts.json
Added diesel engines to vehicle_parts.json ( top level )
Added diesel fuel tanks to vehicle_parts.json ( top level )