Pre-merge into core tasks

[Cadair]

Provide a general description of the issue or problem.

There seems to be general consensus that merging this package into core would be worthwhile. Discussion about if this is a good idea or not is here: sunpy/sunpy#8439.

This issue is to collate any tasks that should be undertaken in the codebase before this merge. The timeline for this merge would be in the next 3-5 months, i.e. in plenty of time before the 8.0 release.

[Cadair]

@hayesla Do you have anything to kick this off?

[Cadair]

My first one: mark tests which require online as remote-data and try and up the offline coverage.