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

homeassistant/util/volume.py

@@ -0,0 +1,45 @@
+"""Volume conversion util functions."""
+
+import logging
+from numbers import Number
+from homeassistant.const import (VOLUME_LITERS, VOLUME_MILLILITERS,
+                                 VOLUME_GALLONS, VOLUME_FLUID_OUNCE,
+                                 VOLUME, UNIT_NOT_RECOGNIZED_TEMPLATE)
+
+_LOGGER = logging.getLogger(__name__)
+
+VALID_UNITS = [VOLUME_LITERS, VOLUME_MILLILITERS, VOLUME_GALLONS,
+               VOLUME_FLUID_OUNCE]
+
+
+def __liter_to_gallon(liter: float) -> float:
+    """Convert a volume measurement in Liter to Gallon."""
+    return liter * .2642
+
+
+def __gallon_to_liter(gallon: float) -> float:
+    """Convert a volume measurement in Gallon to Liter."""
+    return gallon * 3.785
+
+
+def convert(volume: float, from_unit: str, to_unit: str) -> float:
+    """Convert a temperature from one unit to another."""
+    if from_unit not in VALID_UNITS:
+        raise ValueError(UNIT_NOT_RECOGNIZED_TEMPLATE.format(from_unit,
+                                                             VOLUME))
+    if to_unit not in VALID_UNITS:
+        raise ValueError(UNIT_NOT_RECOGNIZED_TEMPLATE.format(to_unit, VOLUME))
+
+    if not isinstance(volume, Number):
+        raise TypeError('{} is not of numeric type'.format(volume))
+
+    if from_unit == to_unit:
+        return volume
+
+    result = volume
+    if from_unit == VOLUME_LITERS and to_unit == VOLUME_GALLONS:
+        result = __liter_to_gallon(volume)
+    elif from_unit == VOLUME_GALLONS and to_unit == VOLUME_LITERS:
+        result = __gallon_to_liter(volume)
+
+    return result