demux publish output on only one selected output topic

[lucasw]

There could be different modes:

• the default stays the same for legacy users
• iterates through all outputs so each gets 1/N of the messages (this is what I want the nodelet to do).
• has a numerical (or string match the topic name?) input topic that selects which output topic to be used (which is most like a real demux). This could be more complicated if it were a mask, so any combination of output topics could be used used (0xff is the default and all outputs get published, 0x1, 0x2, 0x4 and so one gets published, and everything in between with 2 or 3 or 4 etc get published.