Add small dataset for testing.

test-data/README.md

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+Files here are small mock-files for testing.
+
+Each file should have a code-snippet that was used to create if applicable.
+
+# Nexus Files
+## small_coda.hdf
+
+```python
+# In a notebook cell.
+!cp 443503_00031010.hdf copied_coda.hdf
+import h5py
+
+with h5py.File('copied_coda.hdf', 'r+') as f:
+    instrument_gr = f['entry/instrument']
+    keeping_names = ['name', '_publication']
+    for name in instrument_gr:
+        if name not in keeping_names:
+            del instrument_gr[name]
+
+    # Copy the rest of the file
+    with h5py.File('small_coda.hdf', 'w') as new_f:
+        # copy everything
+        f.copy('entry', new_f)
+
+```
+
+## small_ymir.hdf
+
+```python
+# In a notebook cell.
+!cp 876380_00011465.hdf copied_ymir.hdf
+import h5py
+
+with h5py.File('copied_ymir.hdf', 'r+') as f:
+    instrument_gr = f['entry/instrument']
+    keeping_names = ['name', '_publication']
+    for name in instrument_gr:
+        if name not in keeping_names:
+            del instrument_gr[name]
+
+    # Copy the rest of the file
+    with h5py.File('small_ymir.hdf', 'w') as new_f:
+        # copy everything
+        f.copy('entry', new_f)
+
+```