plot utility improvements/retrieving a subset of observations from AdlerPlanetoid

[jrob93]

plot_errorbar currently accepts planetoid and all its observations. We need a way to plot only a subset of observations. E.g. in a plot of reduced mag vs mjd we may want to plot only a single observation.

Maybe just change planetoid before it is passed to plot_errorbar.

[astronomerritt]

Potentially need a method like AdlerPlanetoid.subset() to allow easy subsetting/filtering of data, exclude observations above a certain uncertainty, etc.

Or possibly a method which outputs an Observations object as a DataFrame so filtering can be done that way? science_utilities.py has a get_df_obs_filt() function.

I'm going to think about this!