Add helper functions for evaluation

[jeremykubica]

Add a bunch of functions based on @wilsonbb evaluation notebooks. These include

• A function to do a linear fit from lists of pixel locations and times: fit_trajectory_from_pixels
• A function to compute the mean squared error from a given trajectory and lists of pixel locations: evaluate_trajectory_mse
• A function within WorkUnit that will convert (RA, dec) positions into pixel coordinates: get_pixel_coordinates

Co-authored-by: Wilson Beebe wilsonbb@users.noreply.github.com