Support for gzipped files

[juhoinkinen]

Gzip is a common compression format, but modelscan currently supports only scanning zip compressed files. There are some references to gzip in the code, here and here, but at least I did not manage to scan gzipped files.

Modelscan could detect, read and scan also gzipped files. The detection could utilize the magic numbers for gzip or just .gz filename suffix.

Uncompressing the gzipped files is ofcourse an alternative, but the gzipped files are so common that native support for them would be nice.