[BUG] - Value Error when exporting Serpent format with UCARD

[shimwell]

Describe the bug

the SerpentFormat attempts to use number formatting on a string with this section

{"void":<5d}

of this line on the main branch
Line in code on main branch

GEOUNED/src/geouned/GEOUNED/Write/SerpentFormat.py

Line 148 in 76ef697

f'cell {index:<5d} {self.Options["Universe"]} {"void":<5d} '

This crashes the code with the error message

ValueError: Unknown format code 'd' for object of type 'str'

To Reproduce

Steps to reproduce the behavior:

Set UCARD to a value (not None) and export to the serpent format

Please complete the following information):

• OS: WSL
• GEOUNED version dev and main branches
• Python version 3.11