Draft: Wannier bands

This MR allows the user more freedom to specify the 'fixed window' in the Wannier module. The fixed window may be specified either by a (contiguous) set of bands, by an energy range, or in the old way.

This allows one to describe even large systems with only a few Wannier functions if one is only interested in a smaller subset of bands. In many applications, one is primarily interested in a small set of bands e.g. near the Fermi level. In the current implementation, however, all bands below a user-defined energy threshold must be included in the 'fixed window' part of the Wannierization procedure. This includes all semi-core states, which one typically is not interested in.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.