Incorrect optional declarations in bda docs

[landmanbester]

• Codex Africanus version: latest
• Python version: 3.7
• Operating System: ubuntu 18.04

Trying to use the bda averaging and ran into this. The bda docs declare uvw, chan_freq and chan_width as optional but the function falls over if these are not provided. Mostly you see errors along the lines of

'NoneType' object has no attribute 'dtype'

because they are None by default.