Allow re-use of column specifications across templates

It would be great if one wouldn't need to specify the same column over and over again across many templates.
Therefore I think it would be better if columns were single modular components.

The issue names reusable column specifications and their use across templates, but no files or tests. Start by locating the existing template and column definitions, then determine how shared components should be represented; done means the same column can be defined once and reused across multiple templates without duplication.

Written by the indexing model from the issue text.