digital: fixed narrowband channel model adder to take a signal amplitude to determine the SNR.
digital: changed modulation_utils2 to modulation_utils and update all files using it.
digital: sneaking in a fix to cpm.py here. Working with benchmark_tx. Also adding some print statements to benchmark script to indicate what type of source/sink is being used.
digital: I didn't think this was needed, but apparently, it is.
digital: moved narrowband examples to their own directory (to make room for OFDM examples).
