Add a first test for the CLI

tests/test_climat2.py

+import unittest
+import subprocess
+import os
+
+
+class TestHelp(unittest.TestCase):
+    def test_help(self):
+        proc = subprocess.Popen(['./main.py', '--help'], stdout=subprocess.PIPE)
+        stdout, _ = proc.communicate()
+        self.assertIn(b'usage: main.py [-h] [-c] [-l] [-s] [files [files ...]]', stdout)