Ensure docstrings specify optional outputs

[jwaiton]

As discussed in #10, docstrings should specify when outputs are optional. Optional arguments for functions are already documented within the function calls. An example:

def read_binary(file    :  BinaryIO,
                wdtype  :  np.dtype,
                counts  :  Optional[int] = -1,
                offset  :  Optional[int] = 0) -> np.ndarray:

Docstrings need to be checked to match the expected output, but with optional flagged as such:

    Parameters
    ----------

        file    (BufferedReader)  :  Opened file
        wdtype  (ndarray)         :  Custom data type for extracting information from
                                     binary files
        counts  (int, optional)   :  How many events you want to read in. -1 sets it to take all events.
        offset  (int, optional)   :  Offset at which to start reading the data. Used for chunking purposes
                                     and so should by default be set to zero if not chunking.

    Returns
    -------
        data  (ndarray)  :  Unformatted data from binary file

    '''