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.

tests/README.md

@@ -0,0 +1,43 @@
+# BioxelNodes Unit Tests
+
+This directory contains unit tests for the bioxel module.
+
+## Test Data
+
+Test files should be placed in `tests/data/`:
+
+## Running Tests
+
+Run all tests:
+```bash
+uv run pytest
+```
+
+Run specific test file:
+```bash
+uv run pytest tests/test_bioxel.py
+```
+
+Run specific test class:
+```bash
+uv run pytest tests/test_bioxel.py::TestDataClass
+```
+
+Run with verbose output:
+```bash
+uv run pytest -v
+```
+
+Run with coverage:
+```bash
+uv run pytest --cov=bioxelnodes.bioxel --cov-report=html
+```
+
+## Test Categories
+
+- **TestDataClass**: Data class initialization and properties
+- **TestDataToLayers**: Data.to_layers() method
+- **TestUtilityFunctions**: Utility functions
+- **TestLayerClass**: Layer class functionality
+- **TestConstants**: Module constants
+- **TestEdgeCases**: Edge cases and error handling