Option to treat samplers as one album when calculating album gain

[rsplaul]

The current implementation seems to rely on identical ARTIST and ALBUM tags to recognize files as belonging to the same album. Obviously this doesn’t work well for samplers (and things like Split-EPs for that matter). Tagging the tracks with a mutual ALBUMARTIST isn’t a real solution either.

I would suggest a command line option for replaygain that switches off the auto-detection and forces all tracks to be considered being from the same album.

For collectiongain a command line option could be introduced that forces tracks in the same directory to be considered being from the same album.

An alternative would be an 'light' version of the detection, that only looks for matching ALBUM tags. This would however lead to problems when iterating through big collections with collectiongain, because bands don’t seem to have a agreed on using world-wide unique album names. ;-)