implement: channel write generalization

[eackermann]

Currently the channel write functionality in utils is a little restrictive. In particular, we need to support different formats:

# re-estimate number of packets
num_packets = len(ts)
my_ch_struct = Struct('<%dh' % num_packets)
my_ts_struct = Struct('<%dI' % num_packets) # ts_dtype should affect this!!!!
ts_packed = my_ts_struct.pack(*ts)

but other generalizations are necessary, too.