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

[jonnyh404]

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.