Fill Obs with reduced mangitude

[mkelley]

An idea from @mschwamb

This is a request for

• a new feature
• an enhancement to existing sbpy functionality
• somethings else: [explain here]

The requested changes will be implemented by
Maybe the LSST SSSC?

High-level concept
Implement a method that can compute reduced/absolute magnitude for an Obs object with photometry and ephemeris information.

Explain the relevance to sbpy
Very much a solar system problem.

Proposal details
H = m - 5 log10(rh * delta) + 2.5 log10(Phi(phase))

Example (pseudo-)code
Just pitching a possible API, based on the Ephem.fill_delta_and_phase method proposed in PR #339:

Obs = ...

Obs.fill_absolute_magnitude()  # for H(1,1,alpha)

phase_function_model = HG(G=0.19)
Obs.fill_absolute_magnitude(phase_function_model)  # for H(1,1,0)