Update inventory PO item fulfillment

The SourceLocationId must not equal the WarehouseLocationId for the PO.

Note: Changing the SourceLocationId of a fulfillment step will also change the DestinationLocationId of the previous fulfillment step (if one exists) to the same value.

Note: Changing the DestinationLocationId of a fulfillment step will also change the SourceLocationId of the subsequent fulfillment step to the same value, with two exceptions:

1. The DestinationLocationId of the last fulfillment step cannot be changed since it must always equal the WarehouseLocationId of the PO.

2. Similarly, if the DestinationLocationId of any fulfillment step other than the last one is changed to the WarehouseLocationId of the PO, then all subsequent steps will automatically be deleted.