Rename sale `package_weight` to `weight`

[prakashpp]

package weight field is misguiding[1], we have already made similar change to shipment and package model. With this downstream modules also need to be change.

[1] https://github.com/fulfilio/trytond-shipping/blob/develop/sale.py#L22

[sharoonthomas]

@prakashpp we should deprecate this step by step. As a first step add another field named total_weight and the have the getter of package_weight issue a deprecation warning.