Remove constructors with loudness

[sdatkinson]

These result in a lot of copied code. Instead, we can ->SetLoudness() once at the end of get_dsp().

[mikeoliphant]

👍

This has bugged me since the loudness was added...

[sdatkinson]

Yeah, apologies for this.

I know what higher-order function I want conceptually, but I'm way behind in my C++ compared to how I know how to do it in other languages 😅 (which is my general excuse for a lot of the kludges in this whole project's code!)

But this'll work better than current, which is at least something 🙂