[mb-style] RFC: Allow releases to appear in multiple release groups

[Alex Mauer]

As the ticket I entered[1] was changed to request it, here is the RFC to 
allow a release to appear in multiple release groups.

The ticket says:

There are a few cases where would be handy to allow a release to belong 
to two or more release groups.

The cases I can think of:
* double-album re-releases, where a previously-released album is 
re-released in combination with another previously-released album 
(Commonly 2 records released on one CD, or two records on one 
reel-to-reel tape)
* Alternate EP-length releases of an LP album[2], to allow them to be 
listed under the artist's EP list as well as in the relevant album RG.

This RFC will expire on 2011-9-2

This topic was previously discussed in the thread “RFC: Include 
double-album rereleases in ‘compilations’”[3]

1. http://tickets.musicbrainz.org/browse/MBS-3267
2. http://musicbrainz.org/release-group/f66cb5a9-9830-4158-abfc-c5c69cc3f6a5
3. http://thread.gmane.org/gmane.comp.audio.musicbrainz.style/12015