Improved send support

[steeltrack]

• Unified sends with other mixer attributes like mixer and panning so full get, set and listen support is added. This required some special handling in those mixer functions because of the extra send index
• Updated send to sends to better reflect Ableton's API
• Updated send test
• Updated README
• Resolved conflict with send_id being added in response in a recent commit

[steeltrack]

@ideoforms Just making sure you saw this PR as well. Thanks!

[f-f]

I've been running this for the past week and it works well for me

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