Azimuthal averaging with density

[ttricco]

Azimuthal averaging of density doesn't work when density isn't an explicit column in the SarracenDataFrame.

In the case of rendering / interpolation, it will construct a numpy array of the density using the smoothing length and particle mass.

However, in this case, we use groupby on the dataframe to sort particles into their radial bins, and this approach won't work with a reconstructed numpy array of density.

Related to #87