Having pw schema XML support all valide disk_io options

[unkcpz]

fixes #908

[unkcpz]

Okay, thanks change it back. I test locally and works all good.

[bastonero]

Passing by I'd suggest putting a reminder in the Wiki about this for future scheme releases. And maybe a test with a xml produced with a calculation specifying the disc_io with one of these options? (probably overtesting though)

[sphuber]

And maybe a test with a xml produced with a calculation specifying the disc_io with one of these options? (probably overtesting though)

This is what @unkcpz added originally, but this will only protect against us accidentally modifying existing schema (specifically the one that is tested against) which is extremely unlikely, and it won't help against us adding a future schema that might not have the fix. So I suggested to leave the test as is.