[CLEAN] product and various: light model / code cleaning

Main changes summarized

product
- [REM] weight_net field. Weight net is removed from the product model.
Indeed a product should be modeled without its possible packaging data.
Moreover in the codebase the weight_net field is not really used.
- unused import
- removed _common.py file holding wrappers for tools (rouding and
ceiling), used only once in the codebase

purchase
- unused imports

purchase_requisition
- dead code
- purchase_requisition: from boolean to selection, because V > M

stock
- [REM] qty_available2, logic, loc_rack, loc_row, loc_case fields. location_id
and warehouse_id moved from the product to the template to ease the
configuration.
- [ADD] description_picking field

stock_account
- [REM] date_account field on stock.inventory

misc wording (sale_margin, ...), notably Cost Price renamed to Cost.

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