Add unique decorator to UnitEnum in units.py

The 'unique' decorator from the enum library was added to the UnitEnum class to ensure uniqueness of enum members. This change helps to prevent potential issues arising from duplicate enum values in the future.

src/mdio/schemas/base/units.py

@@ -2,12 +2,14 @@
 
 
 from enum import Enum
+from enum import unique
 
 from pydantic import Field
 
 from mdio.schemas.base.core import StrictCamelBaseModel
 
 
+@unique
 class UnitEnum(str, Enum):
     """An Enum representing units as strings, from pint."""