Allow backwards compatibility when setting float parameters to strings.

[psobot]

This PR fixes an introduced in #167; existing code that may set VST parameter values to strings (i.e.: plugin.frequency = "123.45 dB") will now throw a ValueError due to the fact that Pedalboard now (as of v0.6.5) expects a float instead of a str.

To fix this, Pedalboard now:

• more reliably detects when a VST or AU plugin's parameters have suffixes but don't report them to the host via the .label parameter
• allows setting a float-based parameter with a str
• allows setting a float-based parameter with a str that has its .label suffixed on the end
• exposes a .units parameter on AudioPluginParameter instances, for better discoverability than .label.