Provide complete PFI mm <-> MCS pixel coordinate transformation object

pfs_utils.coordinates holds the coordinate transform functions based on the optical model. We now have measured cobra and fiducial fiber positions through the full system, and understand the additional terms better.

Please provide an object which accounts for all known distortions, and which can transform MCS pixel <> PFI mm (and eventually bench camera pixel <> PFI mm). We will also need to transform cobra arm lengths.