Improve organisation of readers & way in which we store init arguments

[IAlibay]

Follow up from #3685 - in order to fix copying of readers we have a rather cumbersome decorator in place. As @lilyminium mentioned it might be possible to get this to work via metaclass magic, which would definitely lower the burden on remembering to add it in on future readers.

Additionally, the way in which we test for copying is via MDAnalysisTests/coordinates/test_copying. However new readers have to be added in manually (which we often forget to do) and readers with optional dependencies have to be dealt with separately. I would recommend we move test_copying to the base reader tests so that when we eventually move all the readers to use them, we won't have this issue anymore.