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/bioxel/layer.py

@@ -1,6 +1,12 @@
 import copy
 import numpy as np
 
+# 条件导入 openvdb（只在 Blender 环境中存在）
+try:
+    import openvdb as vdb
+except ImportError:
+    vdb = None
+
 from . import scipy
 from . import scipy as ndi