This adds two new parameters for modeling of nodal and apsidal precession: NodalPeriod and ApsidalPeriod. The default unit for both is years.

Example syntax (orbit data from 16 Cyg Bb):

	EllipticalOrbit
	{
		Epoch 2456937
		Period 2.18863792
		SemiMajorAxis 1.6795
		Eccentricity 0.683
		Inclination 137.41309
		AscendingNode 316.95447
		ArgOfPericenter 232.8624
		MeanAnomaly 1.03341
		NodalPeriod 100
		ApsidalPeriod 100
	}

For the following images, I've set OrbitPeriodsShown to 3.0 in celestia.cfg.

Nodal precession:

Apsidal precession:

One issue is that the orbit paths look incomplete (this is with fading disabled). I wonder if using some parameter other than period (such as eccentric anomaly) for sampling the orbit would address that.

I'm not sure if the equations used are correct either.