Added an optional condition variable for notifying external control loops

[jontje]

As per title.

This can for example be useful when having multiple communication channels. I.e. for getting a notification that any of the channels have received an EGM message.

[gavanderhoorn]

Can you explain a little more about how this is supposed to be used by users of this library?

Exposing variables is not something I typically like to do. Could a callback based interface not be used?

[jontje]

Can you explain a little more about how this is supposed to be used by users of this library?

This PR adds the possibility to use a condition variable, for which a user can get a notification on that an EGM message has arrived on a given EGM communication channel.

Example:

• A user have 2 EGM channels (one for robot A and one for robot B).
• The user just wants to wait for a message on any of the channels.
• The waitForMessage(...) function could be used for each EGM channel, but it will have problem if one of the EGM channels have been disabled (which might be ok, depending on the application). Then the disabled EGM channel's waitForMessage(...) would unnecessarily block the execution thread.
• A condition variable could instead be shared between the EGM channels, which then both can send notifications on.

At least this is the idea, and I hope I my description isn't too unclear.

Exposing variables is not something I typically like to do. Could a callback based interface not be used?

I guess, but I will unfortunately not have time to rework this right now due to time constraints.

The optional condition variable felt like a minimal intrusion.