The model `procurement.group` is replaced by a more flexible one:
`stock.reference`. The first one was somehow not well understood and a
bit limited by its many2one links to other business model (Sales Order,
Purchase Order, ...). The reference is made of a name and many2many to
all the documents integrated with Inventory. It's purpose has been
shrank to the minimal "make links between object". For instance
opening the manufacturing order created for a MTO sale. The merge of
stock move is delegated to existing parameters (`sale_id`,
created_purchase_line_ids`, ...)

The `mrp.production` needed an extra effort to group them according to
inheritance relationships. This commit add a new object to group
productions together either in a backorder relation or in a child-parent
one. This allow for instance to have a Sale Order linked to the MTO
product of the sold product and the production them selves to be linked
to the SO *and* the potential child MOs.

The procurement group propagation, before set on the Stock Rule, allowing
the stock moves to be merged in an existing picking or not has been
removed and replaced (for the creation of purchase order only) by a new
parameter on `res.partner` allowing the group of Request for Quotation
base on the scheduled date of the potential merge candidate (never, same
week, same month, always). All other merge scenarios should use batches
and waves transfers.

task: 4279803