Allow `SEDPlotter.plot()` to change units

[HealthyPear]

Feature Request

Is your feature request related to a problem? Please describe.
Allow SEDPlotter.plot() to change units

Describe the solution you'd like
add a new kwarg yunits and xunits to the call of plot_sed and propagate it to plot

internally convert before plotting with astropy.units

[omodei]

Sounds like an interesting feature to add. @HealthyPear can you please work on it on a separate branch and submit a PR when implemented? We will be happy to review it and merge. Thanks!