Convert get_supply_period to classmethod to improve generate_requests() performance
The Production.generate_requests() method calls the Product.get_supply_method() for each product, but the value is allways the same because it comes from production configuration.
We have an instance with 27.000 products. This call takes 0,004 seconds that means nearly to 2 minuts to get this constant information.
The whole generate_requests() for 27.000 products and supply_period = 0 takes 160 seconds.
With the classmethod version of get_supply_periods() (which also return value for each product) it goes down to 15 seconds.