Add Time Lag model for Gas Giant Dissipation

[jrenaud90]

Implement a simplified global rheology for gas giant dissipation.

Based on Hussmann & Spohn 2004:
-Im[k2] = k_J (t) / Q
k_J (t) = k_J (t_0) * (1 - n_io(t) / spin_J) / (1 - n_io(t_0) / spin_J)

n_io = Io's mean orbital motion
spin_J = Jupiter's spin rate (H&S assumed constant over time)
k_J(t_0) = Jupiter's modern k2 = .379
Q = Jupiter's fixed Q. H&S let this be a free parameter >= 6.6e4

• Is this still an accepted estimate for gas giant dissipation to first order?
• Can this be easily applied to other gas giants (Saturn) that have a dominant dissipating satellite (Titan?? Enceladus??)