Add unit tests for bioxel module and documentation

- Created README.md for test instructions and categories.
- Implemented comprehensive unit tests for the Data, Layer, and utility functions in the bioxel module.
- Added tests for reading NRRD and DICOM formats, data properties, and edge cases.
- Included tests for real data files to ensure functionality with actual datasets.

src/bioxelnodes/node.py

@@ -105,7 +105,7 @@ def add_bioxel_node(name: str):
         bpy.ops.object.modifier_add_node_group(
             asset_library_type="CUSTOM",
             asset_library_identifier="O Bioxel",
-            relative_asset_identifier=f"Nodes.blend\\NodeTree\\{name}",
+            relative_asset_identifier=f"BioxelNodes.blend\\NodeTree\\{name}",
         )
         bpy.ops.object.modifier_remove(modifier=name)
         try: