Fix EUI observer coordinate date

[dstansby]

Fixes #5450. Currently I've just passed a custom value to .observer_coordinate property, which means that .observer_coordinate.obstime != .time, to avoid changing the return value of .time.

[ayshih]

Does this mean that .coordinate_frame.obstime != .coordinate_frame.observer.obstime? (This isn't necessarily a problem, but is better avoided if possible.)

[dstansby]

Does this mean that .coordinate_frame.obstime != .coordinate_frame.observer.obstime? (This isn't necessarily a problem, but is better avoided if possible.)

Yes, it does, because .coordinate_frame draws a date from .wcs, which in turn draws from .date.

The other way to go with this is to override .date to return DATE-AVG instead of DATE-OBS, which might be better for overall consistency? (and was suggested by @wtbarnes here: #5450 (comment))

[wtbarnes]

I'm more inclined to override .date. However, I'm not really bothered either way as this mostly matters for bugfix releases on 3.0.x as #5449 will solve this problem by returning DATE-AVG from .date anyway.