GRIS Coordinates without cross correlation
In general we want to have a full cross correlation of coordinates fro GRIS maps. This is the preferred method of getting coordinates for WCS keywords. Back when we did this, we assumed SLITPOS_X and SLITPOS_Y to give coordinates in a system that is aligned with the solar axis and for that matter with the HMI system.
Looking at the location guesses from the Mercury transit of last year, it seems very likely, that there is an additional rotation between the systems that we don't account for. This would also affect our uncertainty estimates for non-correlated files. See #199 (closed)
