'operation' parameter added to bus_add

allows choice of addition or subtration
Renamed bus_add to bus_addsub

Changed operation parameter specification so can choose add or subtract
separately for each bus component. Renamed to addsub
Twiddle modification for multiple inputs and async

* twiddle blocks fully scripted
* parameter option to process more than one stream (number inputs)
 (still need to make bus_negate to complete twiddle_stage_2 and twiddle_coeff_1)
* output a and b (instead of a_re, a_im, b_re, b_im) as will use bus_addsub
  blocks that can take complex inputs).

Note that the fft will not work yet, butterfly_direct needs to be modded to
use these new twiddles in future commmits. This should actually be on a branch.