bom的类型
'type': fields.selection([('normal','Normal BoM'),('phantom','Sets / Phantom')], 'BoM Type', required=True,
help= "If a by-product is used in several products, it can be useful to create its own BoM. "
"Though if you don't want separated production orders for this by-product, select Set/Phantom as BoM type. "
"If a Phantom BoM is used for a root product, it will be sold and shipped as a set of components, instead of being produced."),
如果虚拟对bom用于一个顶层产品。销售装箱时 会直接用bom组件 替代 销售产品，而不创建生产单据。
虚拟的bom用于不要生产加工的产品组合： 例如，几个产品打包销售，a+b+c 组合成D销售，
销售D时，并不需要用a b c 来生产D， 这种情况，为D 建立一个 phantom BOM。 销售D，会直接生成abc 的送货单。