Allow user to specify center frequency and bandwidth in user interface

[aaronwoodcox]

As a user, I would like to be able to specify a desired center frequency and bandwidth in the UI instead of needing to specifying start and stop frequencies. Selecting a center frequency and bandwidth would improve usability and bring the interface much more inline with commercially available spec-ans. (For consideration: Perhaps the ability to specify start and stop frequencies could be retained, but then augmented with the ability to also specify center freq and bandwidth. And when a user updates one set of fields the UI could automatically compute and update the corresponding - i.e., if user selects center freq and bandwidth then the start and stop freqs could be calculated and shown in the UI and vice-versa.)