Suggestion to make your PID-code easier to use as a library

[rpoisel]

Hi Ruben,

thanks for publishing your PID-library! It seems to be very useful to me.

This pull-request is merely a suggestion on how to make your code easier to use as a library. Instead of pulling other code (e. g. examples) in as git submodules, I would do it the other way round: using this git repository as a submodule somewhere else (e. g. a git repository with examples using this library).

Furthermore, I extracted the tick-handling functions so that it is possible as a user of the library, to provide your own implementation. This is a requirement in my case.

• PID_Support.h is the include file to be included by the code providing the implementation of the tick-handling functions. It is also included by PID.c, because it will then call these functions.
• PID.h is included by users of this library. They necessarily do not need to know anything about tick-handling functions.

Please don't hesitate to get in touch with me in case you need any further information or in case you have any other suggestion.

Rainer