Environment variable to determine the default output file

[zerothi]

Describe the feature
The problem is that multiple codes might have defaults for the files even if they have the same suffix/prefix.

We need some way of handling this.

[zerothi]

@tfrederiksen this might interest you.

Before the sisl siles were kind-of forced to have the same suffix. This is now changed.

There are now multiple codes using the out suffix, and detection can be streamlined using SISL_IO_DEFAULT=siesta for instance.

The basic principle can be seen here: docs/api/io/index.rst in da3c29b. Let me know if the documentation is clear enough.