Writes the light curve to a CSV file.
This method will convert the light curve into the Comma-Separated Values
(CSV) text format. By default this method will return the result as a
string, but you can also write the string directly to disk by providing
a file name or handle via the path_or_buf parameter.
File path or object. By default, the result is returned as a string.
Dictionary of arguments to be passed to TimeSeries.write().
Returns a csv-formatted string if path_or_buf=None.
Returns None otherwise.