Add option to have separate SDR and transmission frequencies

It would be good to add handlers to python/hw_settings.py that return the frequency at which the SDR operate, and the frequency at which the Doppler correction is applied. For most users, these will be identical, but some users may operate a local oscillator (for higher frequency observations), and having these handlers will allow them to adjust SDR frequency handler to offset it by the local oscillator (e.g. pjm on S-band).