[UPD] improve related_dispatch_ids fnct_search function

linked to @guewen advices on #20 (thanks!):
- improves fnct_search function
- delete deprecated "method=True"

Oh! Ok, it makes sense. Still, 'in' and 'not in' could be added since they would also be handled properly if a list of ids is passed (example when doing a direct search on the field from a python method).

In fact, after thinking, I think that the restriction could be removed entirely: the method only dispatches the search from a model to another, so there is no reason to restrict the operators, the search() on stock.move will eventually do.