'Retrieves the unit attribute name for the given unit string.
For example, if the given unit string is \'metre\', \'m\' would be returned.
An exception is raised 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))
