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.

.gitignore

@@ -135,4 +135,6 @@ pythonlib*
 # Blender
 *.blend1
 blendcache_*
-*.cats.txt~
+*.cats.txt~
+
+tests/data