Candidate implementation

Hi,

Here's a rather straightforward candidate implementation for pid-config. As discussed, it doesn't require any modification on the PID side and it only stores a single copy of the parameters.

What I don't like about this implementation : the extern function pointers included in the .c file for testing.
I've been thinking about it and couldn't figure out another way to abstract the interface for testing.
Forgive me if I've missed something obvious there, I'd be happy to change it.

Both unit tests (pid-config only) and integration tests (with pid module) are included.

Cheers,

Pierluca
your API doesn't specify that timeout needs to be in us! Also we decided to use SI units everywhere (except for ints where it isn't possible of course).