Draft of the Generic IO protocol extension of the Simple Message protocol

As per title.

It is probably easier to read the document rendered, you may use [this](https://github.com/gavanderhoorn/rep-ros-i/blob/rep-xxxx_generic_io/rep-ixxxx.rst) url for that. I've also pushed a PDF rendered with [restview](https://mg.pov.lt/restview/): [link](https://github.com/gavanderhoorn/rep-ros-i/raw/pdf_attachment/rep-ixxxx.pdf).

Comments may be added in-line here (on the commit), or at the bottom of this PR.

Some points for discussion:

 1. should ``seq`` field of ros-i header be used instead of ``message_id``
 1. should copied ROS Packet header structure be shown in more detail in this document (seeing as the original source doesn't explicitly give more detail on used types fi)
 1. should message structures be defined using `shared_int` and `shared_real`, instead of current C typedefs
 1. should a `name` field be added to `IO_INFO` ranges
 1. repeat `timestamp` field in `IO_STREAM_PUB` for each published range, instead of one per msg
 1. should streaming support be required
 1. should explicit support for IO timed with motion be added (ie: synchronise IO write/read execution with `time_from_start` in trajectories), or should that be a responsibility of the client

Is this required?  Is it just simpler to say requests will be handled and returned in the order in which they are received?  On the controller side, this may be all that is allowed given the simplicity of the language.  Even those controllers with multi-thread support, have a limited number of threads.