--- handles the unlockable market items from Config.lua in map_gen.Diggy
-- serves as a handler for an array of items and passes it on to FormatMarketItems.add() that returns the correct format for Diggy.Feature.MarketExhange.lua to process.
-- @param items array of items where each item is an table with keys: level (integer level it unlocks at), price (price in the configured currency_item) and name (has to be an entity's prototype-name)
-- @returns table of items formated in the correct way for Diggy.Feature.MarketExhange.lua to interpret.