Volume feature

[Inglebard]

Hi,
I don't know if it's the best way to do that, but i think the possibility to change the volume can be interesting.

[Stealthii]

This is similar to the volume work I did on the original PiFM C code. I would suggest squashing these commits into a single commit and giving it a better description.

I have tested this code and it works as expected.

[ChristopheJacquet]

Hi,

Okay to add the feature. Thanks for the patch, however it decreases the amplitude of the whole MPX signal, i.e. not only of the audio component, but also of the RDS component and pilot tone. I suppose this can be a problem. I'm going to look into that.

[Inglebard]

Hi,
Be careful i have not a lot of skill in C/C++ programming.
Like Stealthii said, I found a note on original Pifm code and i need it in PifmRDS (to much volume diffence between original radio station and PifmRDS), so i tried to apply and update for PifmRDS.
The "patch" may be not correct/not optimise.