Added proper NSZ handling

[topiga]

Pull request type

• [x] Bugfix
• [ ] Feature
• [ ] Documentation/Content changes

What is the current behavior?

When splitting an NSZ file, the file is handled perfectly, but the extension is still ".nsp". It is frustrating to always having to change the file extension.

Issue Number: #15

What is the new behavior?

• The script now detects if a file is an .nsz file, and changes the output automatically
• The script now puts a ".nsz" extension if the original file is an NSZ file