Simulate guiding and tracking errors

[dkirkby]

This is a feature request to simulate throughput variations during an exposure due to individual fibers not being centered on their targets. The initial plan has two ingredients:

• Add a random jitter parameter that effectively convolves the focal-plane PSF with a Gaussian.
• Use the methods of http://arxiv.org/abs/1506.04790 to calculate time- and wavelength-dependent throughput variations under ideal guiding conditions.