Mestel ILR failure in OrbitalElements v2.0

[michael-petersen]

This produces an error:

using OrbitalElements
n1,n2 = -1,2
model = MestelPotential()
resonance = Resonance(n1,n2,model)

(as does this)

using OrbitalElements
n1,n2 = -2,3
model = MestelPotential()
resonance = Resonance(n1,n2,model)

This seems to be related to needing to find a maximum along the \beta_{circ} curve to set resonances.

Neither case fails if model = TaperedMestel().