Replaced GCM functions with ISO functions

Renamed various commands to refer to ISO files instead of GCM files for consistency,
as the commands are used for both Wii and GameCube files.

CompressGCM --> CompressISO
DeleteGCM --> DeleteISO
MultiCompressGCM --> MultiCompressISO
MultiDecompressGCM --> MultiDecompressISO
SetDefaultGCM --> SetDefaultISO