'Retrieve the unit attribute name for the given unit string.
For example, if the given unit string is \'metre\', return \'m\'.
Raise an exception if an attribute cannot be found.'

@classmethod
def unit_attname(cls, unit_str):

    lower = unit_str.lower()
    if (unit_str in cls.UNITS):
        return unit_str
    elif (lower in cls.UNITS):
        return lower
    elif (lower in cls.LALIAS):
        return cls.LALIAS[lower]
    else:
        raise Exception(('Could    not    find    a    unit    keyword    associated    with    "%s"' % unit_str))
