303 documentation or code error for vel frame

[mpound]

• Corrects documentation in velocity frames notebook.
• Changes behavior of spectrum.plot() such that if x-axis is velocity and velocity frame is not given, it defaults to the frame in VELDEF keyword rather than the topographic frame frequency default in the velocity_frame attribute of Spectrum.
• Adds a cell to velocity frames notebook. demonstrating above.

Although this is the desired behavior in the issue report, I am not entirely convinced this is behavior we want, because it is special casing one type of call to plot(). However, if Spectrum.velocity_frame was changed by the user via dysh (e.g. Spectrum.set_frame), then VELDEF will also get changed the behavior should still match user expectations.