Get rid of the wann2kc.w90_files

[mikibonacci]

This can be done by using the self.link(Process, wann2kc_calc) and use the dst_path name to understand the attribute.

then, in the helper used to populate the builder, I use a pop to automatically delete the custom attribute.

[GeigerJ2]

Note: decorate self._fetch_linked_files, not self.link.

[mikibonacci]

Exactly, so the idea would be to decorate not the link but the _fetch_linked_files, which contains the src_calc attribute.