Ensure platform-independent path manipulation

[obo]

Almost all path manipulation operations use string concatenation and / as the directory delimiter.
This is wrong. os.path.join() should be used to build paths and similarly for path splitting.

[mohammad2928]

It's done.

[obo]

Hopefully it's complete, let's mark it closed. :-)

[obo]

Reopening because the new index parser (https://github.com/ELITR/SLTev/blob/index-parser/SLTev/index_parser.py) uses glob.