Port AtoB encoder to AmbiX

[lossius]

[joslloand]

similar to: https://github.com/ambisonictoolkit/atk-reaper/issues/73

We should probably have a discussion about this... whether a using flag for target format would be the way to go.

Relatedly, this probably applies to the complete plugin project...

[mtmccrea]

+1 for the addition of the flag throughout the project, for both input and output formats :)

It would make make the integration with other workflows more seamless, especially given that many modern recorders and mic encoders are outputting SN3D.