blocks: callback function additions for repack_bits , stream_mux and stream_to_tagged_stream;  blocks: smoother added for throttle; digital: callback function added for packet_headergenerator; digital: naturally mapped 8psk and 16qam constellation objects added;  trellis: corrected a missing callback function

Hi all,

The changes are the following:
For gr-blocks: 
1) I added callback functions to both bits per input and output bytes of repack_bits in gr-blocks to make it able to be changed dynamically. 
2) Similarly, I added callback functions to stream_to_tagged_stream and stream_mux to enable dynamical change of parameters.

3) @anastas found there exists huge delay in a system we are working on due to some buffer in the queue. He modified the throttle to decrease the delay by decreasing the samples throttle process at a time. 
So the throttle was modified by adding an optional parameter max_noutput_items.


For gr-digital:
4) I also added callback function set_header_formatter to packet_headergenerator to make the header able to be chaned dynamically.

5) I've also added two constellations objects in the constellation of gr-digital. One of the constellation object is the natural mapped 8psk and the other is a 16qam. These two constellations are both used in trellis coded modulations based on the paper by G. Ungerboeck "Channel coding with multilevel/phase signals", IEEE Trans. Info Theory, vol. IT-28, pp. 55–67, 1982.

6) I found the chunks_to_symbols has callback function set_symbol_table, but it wasn't working! I found it missed the set_symbol_table in chunks_to_symbols.grc and chunks_to_symbols.h, and added const to std::vector@O_TYPE &symbol_table to make it work.

For gr-trellis:
7) I found the trellis_encoder.grc missed the callback function set_K, so I added it to make it work.

I'm sorry that I wasn't making a clean commits history last time to cause the delay in merge. 
Could you check the updated changes and merge them to the gnuradio if possible.

Thanks!
Best,
Zhe