Remove precalculated pixel-to-WCS lookup table

This MR removes the precalculated pixel-to-WCS lookup table which takes huge amounts of memory (two floats per pixel) and slows down image loading in favor of calculating the necessary coordinates on the fly as there aren't that many users in the first place and those mostly just check for one position in the image.

Most complex part is calculating the minimum and maximum RA/DEC values of the image which I've done by calculating the min/max at image edge pixels and checking if either of the poles is in the image, in which case we know the min/max DEC will be +-90. This seems to work fine, but I don't mind if someone tries to punch holes into my reasoning :)