Consolidate cleaning the shared pump outlet line

Currently `dispense_to_vessel` leaves the pump line purged but does not call `purge_pump_line`. That's because the `purge_pump_line` is hardcoded to purge the pump contents to waste.

Let's consolidate code to:
* have `dispense_to_vessel` call `purge_pump_line` under the hood so that we can share the same line-clearing strategy.
* update `purge_pump_line` to be able to purge into various vessels: either waste vessel or the reaction vessel.