Enable BMW component to be unit system aware (#17197)

* Enable BMW component to be unit system aware

* lint fixes

* use constants for config entries

* remove configuration from component and rely only on HA config of unit_system

* remove unused import

* update code to reflect feedback

* lint fixes

* remove unnecessary comments

* rework return statement to satisfy pylint

* more lint fixes

* add tests for volume utils

* lint fixes

* more lint fixes

* remove unnecessary comments

tests/util/test_volume.py

@@ -0,0 +1,49 @@
+"""Test homeassistant volume utility functions."""
+
+import unittest
+import homeassistant.util.volume as volume_util
+from homeassistant.const import (VOLUME_LITERS, VOLUME_MILLILITERS,
+                                 VOLUME_GALLONS, VOLUME_FLUID_OUNCE)
+
+INVALID_SYMBOL = 'bob'
+VALID_SYMBOL = VOLUME_LITERS
+
+
+class TestVolumeUtil(unittest.TestCase):
+    """Test the volume utility functions."""
+
+    def test_convert_same_unit(self):
+        """Test conversion from any unit to same unit."""
+        self.assertEqual(2, volume_util.convert(2, VOLUME_LITERS,
+                                                VOLUME_LITERS))
+        self.assertEqual(3, volume_util.convert(3, VOLUME_MILLILITERS,
+                                                VOLUME_MILLILITERS))
+        self.assertEqual(4, volume_util.convert(4, VOLUME_GALLONS,
+                                                VOLUME_GALLONS))
+        self.assertEqual(5, volume_util.convert(5, VOLUME_FLUID_OUNCE,
+                                                VOLUME_FLUID_OUNCE))
+
+    def test_convert_invalid_unit(self):
+        """Test exception is thrown for invalid units."""
+        with self.assertRaises(ValueError):
+            volume_util.convert(5, INVALID_SYMBOL, VALID_SYMBOL)
+
+        with self.assertRaises(ValueError):
+            volume_util.convert(5, VALID_SYMBOL, INVALID_SYMBOL)
+
+    def test_convert_nonnumeric_value(self):
+        """Test exception is thrown for nonnumeric type."""
+        with self.assertRaises(TypeError):
+            volume_util.convert('a', VOLUME_GALLONS, VOLUME_LITERS)
+
+    def test_convert_from_liters(self):
+        """Test conversion from liters to other units."""
+        liters = 5
+        self.assertEqual(volume_util.convert(liters, VOLUME_LITERS,
+                                             VOLUME_GALLONS), 1.321)
+
+    def test_convert_from_gallons(self):
+        """Test conversion from gallons to other units."""
+        gallons = 5
+        self.assertEqual(volume_util.convert(gallons, VOLUME_GALLONS,
+                                             VOLUME_LITERS), 18.925)