Rename include_file/file_format to layout or structure

[wenzeslaus]

The word format is already used for the file format as in YAML, CSV, XLSX, ...

The file_format key of the special include_file mapping does not specify the above file format. Instead, it specifies internal layout of the file, i.e., how the file contents should be included in the main file. One option is list which means each item in the file a list item. Item in the file is table row (like in the scenario table) or YAML list item. The default option is to simply continue nesting the values, so that's just plain YAML and for tables, it is like the user-friendly format with one value per row.

Implementation:

https://github.com/ncsu-landscape-dynamics/popsborder/blob/main/popsborder/inputs.py#L187

Documentation:

https://github.com/ncsu-landscape-dynamics/popsborder/blob/main/docs/run.md#including-other-configuration-files