Replaced generalized instances of GCM with ISO

Renamed various commands to refer to ISO instead of GCM for consistency,
as the commands are used for both Wii and GameCube files.

CompressGCM --> CompressISO
DeleteGCM --> DeleteISO
MultiCompressGCM --> MultiCompressISO
MultiDecompressGCM --> MultiDecompressISO
SetDefaultGCM --> SetDefaultISO

Fixed COMPRESSISO

Fixed missing "COMPRESSISO"

Fixed more COMPRESSISO

Final fix for COMPRESSISO