phase curve bounds

[jrob93]

By default the sbpy phase curve models have bounds set on the phase parameter, which limits the fitter to these values. We probably want an option to disable these bounds, e.g. for an adler PhaseCurve object pc:
pc.model_function.G12.bounds = (None,None)

N.B. that bounds may be necessary in some cases, e.g. the sbpy HG model and astropy LevMarLSQFitter will fail without bounds when very high phase angle data is included in the fit (see example object ssoid = 8268570668335894776)

[jrob93]

This was closed by #120