Piano example with multiple concurrent notes

[leeola]

So after looking at this example i wanted to try and alter it to support multiple notes simultaneously. Here's an example based on midi_fundsp's implementation of an identical feature. This led me to some questions/thoughts.

1. If all you're using is a single DSP, as this example, the DspManager feels heavy. It had me wondering if an alternate API could exist for single-shot DSPs?
2. If the integration to Bevy was such that the creation of a DSP could use Bevy's Res<T> types, then one could push NoteBuf into a resource, and use that to create a DSP, avoiding the need to create them at startup and clone them on calls to generate_graph. For example, the startup system could just get the Res<NoteBuf> and create a DSP from that. Which is kinda what i wanted to do.. but not sure if it's possible in the current API.

The example could definitely use some tweaks that could be made to how it integrates to both Bevy and bevy_fundsp, but this is just where i decided "good enough" is for now. Though i'm of course interested in any feedback.

Thanks again for the repo, helps a lot!