Pipette should not make unnecessary up-down movements between actions in same well

[umbhau]

As a robot user, I would like the pipette to hold its z-position between two actions in the same well, so that it can complete protocols more efficiently.

Acceptance Criteria

• If robot is performing two sequential actions in the same well, the robot should maintain its z-position (i.e., not raise the pipette out of the well) between the actions.

-- Example: The pipette should remain in the same position when executing the following command, and not move between mixing (one aspirate and dispense) and aspirating its transfer volume:

    pipette.transfer(
        100,
        plate.wells('A1'),
        plate.wells('A2'),
        mix_before=(1, 50))

• As an exception case, the robot should move (raise out of the well) if creating an air gap before or after an aspirate.

[umbhau]

@btmorr

The above should handle Kristin's request for an 'aspirate with prewet' command.

Worth thinking about eventually creating a 'prewet' alias for 'mix_before' (defaulted to a single aspirate/dispense cycle).

[mcous]

Issue closed by #839 (please re-open if I'm wrong here)