Reorganize `src` folder

[puzrin]

• Move phy-related thing to src/phy
• Create separate subfolder for each driver

non-breaking

[manuelbl]

Thanks for the PR.

The organization makes sense.

But are include paths with . and ../ really needed? Can't we replace them with paths relative to src?

• "phy/CMSISHelper.h" instead of "../CMSISHelper.h"
• "PDController.h" instead of "../../PDController.h"
• "phy/PDMessageDecoder.h" instead of "../PDMessageDecoder.h"
• "phy/NucleoSNK1MK1/NucleoSNK1MK1.h" instead of "./phy/NucleoSNK1MK1/NucleoSNK1MK1.h"
• and so on...

The only exceptions would be include paths referring to a file in the same directory. They wouldn't be relative to src.

I made a quick check with PlatformIO and the Arduino IDE for a single board, and it works.

[puzrin]

Ups... that's because my CPP experience is far from perfect, and sometimes I do strange things :). Thank you very much for your recommendation. Updated.