Correct docstring for onehot and make max_length Optional

  It does not need RDKit to be installed to work with arbitrary strings.

  """

  def __init__(self, charset: List[str] = ZINC_CHARSET, max_length: int = 100):

  def __init__(self, charset: List[str] = ZINC_CHARSET, max_length: Optional[int] = 100):

    """Initialize featurizer.

    Parameters

    ----------

    charset: List[str], optional (default ZINC_CHARSET)

    charset: List[str] (default ZINC_CHARSET)

      A list of strings, where each string is length 1 and unique.

    max_length: int, optional (default 100)

    max_length: Optional[int], optional (default 100)

      The max length for string. If the length of string is shorter than

      max_length, the SMILES is padded using space.

    """