erp5_pdm: Fix Supply Cell update when {en,dis}abling sliced pricing.

Deleting matrix cells with manage_delObjects is not a good solution, as
this breaks matrix index consistency.
Also, call isBasePricePerSlice rather than introspecting wrapped method
arguments - the only wrapped method is _setBasePricePerSlice anyway !
Also, explicitly trigger SupplyCell_updateSliceBasePrice as it depends on
isBasePricePerSlice value at line level, and it just changed, and cell-level
interaction workflow does not trigger on line changes.
Also, get rid of a trivial local variable.
Also, fix the exception expected from list.index .
Also, raise if multiple quantity criterion are somehow found.