Coordinate system in simulated focalplane is incorrect

[tskisner]

The simulated instrument model included in instrument_sim.py includes a coordinate system reflection error when projecting wafers on the sky. We should fix this (and document it) in a way that matches the Xi / Eta coordinate system adopted by Simons Observatory. This same error impacts similar code in that project (simonsobs/sotodlib#163), as well as code in repositories used for CMB-S4 and LiteBIRD.