[FIX] mrp: unfuck bom structures views

A previous refactoring brought a bom_line_ids field on the mrp.bom, thus
deprecating the _child_compute method. But the previous refactoring did
not go through all the views, breaking everything that relied on the
_child_compute (tree view, report). As the bom_line_ids refers to the
mrp.bom.line model (introduced by this previous refactoring, note:
_child_compute returned mrp.bom record) and that we can't make a treeview
showing different model, this patch introduce a function field _get_child_bom_lines
on the mrp.line model, allowing to go through the bom_line_ids of a mrp.bom.line
if this mrp.bom.line refers to a mrp.bom.