Simulate guiding and tracking errors

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.

No files, tests, or entry points are named. Start by locating the simulation entry points and reviewing the linked arXiv paper, then determine how PSF jitter and time- and wavelength-dependent throughput should fit the existing model. Done means both requested effects are represented by usable simulation parameters and covered by appropriate validation.

Written by the indexing model from the issue text.