Resolve "Bug when writing a measurement file with no GW file input"

Closes #103 (closed)

• Test measurement-file writing
• Cleanup test measurement file after tests
• Set gw_group to None when no GW file are used as inputs

tests/__init__.py

+#! /usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""Test setup functions."""
+
+import os
+
+
+def teardown_module(_):
+    """Remove test measurement file after all tests are executed."""
+
+    try:
+        os.remove('measurements.h5')
+    except FileNotFoundError:
+        pass