Update Include file paths from relative to absolute when files are moved by write_out_case

**Describe the bug**
Adding a nexus case using `model.write_out_case` does not update the include file paths within a modified file causing issues with relative paths.

**To Reproduce**
Modify a file_content_as_list attribute on a NexusFile.
Set file modified status `_file_modified_set(True)`
call `model.write_out_case(new_file_path=path, new_include_file_location='include_files')`

**Expected behavior**
Expected behaviour is to also update the paths in the NexusFile when writing to the new location to account for the existing files relative paths.