update the volcabass and volcakeys modules for single trigger operation

[robertoostenveld]

At this moment they require one published event (trigger) per note. They should be updated to enable one published event (e.g. the sequencer) combined with a control value for the pitch.

@stephenwhitmarsh note that this does not apply to the volcabeats module, since the different drum sounds don't form a continuum and therefore cannot be meaningfully controlled with a continuous control value. And note that the volcabass and volcakeys don't have MIDI out, which makes them code-wise dissimilar from the keyboard/piano module.