Adds missing __next__ method to SAS7BDATReader needed for iteration

[spillz]

• closes Issue iterating with pd.read_sas pandas-dev/pandas#13654

[benjello]

Is this PR working. If the answer is positive, it would be nice to have it in upstream.

[benjello]

I insert the next method you provided but I ended up the following error

Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
...
Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
Exception ValueError: ValueError('Unexpected non-zero end_of_first_byte',) in 'pandas.io.sas.saslib.Parser.process_byte_array_with_data' ignored
Traceback (most recent call last):
  File "./fonction_publique/scripts/clean_raw_career.py", line 43, in <module>
    sys.exit(main())
  File "./fonction_publique/scripts/clean_raw_career.py", line 38, in main
    chunksize = chunksize,
  File "/home/benjello/openfisca/fonction-publique/fonction_publique/raw_data_cleaner.py", line 202, in main
    chunksize = chunksize,
  File "/home/benjello/openfisca/fonction-publique/fonction_publique/raw_data_cleaner.py", line 117, in format_generation
    generation = get_subset('generation', file_path, debug = debug, chunksize = chunksize)
  File "/home/benjello/openfisca/fonction-publique/fonction_publique/base.py", line 122, in wrap
    ret = f(*args, **kwargs)
  File "/home/benjello/openfisca/fonction-publique/fonction_publique/raw_data_cleaner.py", line 36, in get_subset
    for chunk in reader:
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/io/common.py", line 113, in <lambda>
    BaseIterator.next = lambda self: self.__next__()
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/io/sas/sas7bdat.py", line 585, in __next__
    return self.read(nrows=self.chunksize or 1)
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/io/sas/sas7bdat.py", line 607, in read
    rslt = self._chunk_to_dataframe()
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/io/sas/sas7bdat.py", line 649, in _chunk_to_dataframe
    dtype=self.byte_order + 'd')
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/frame.py", line 2417, in __setitem__
    self._set_item(key, value)
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/frame.py", line 2483, in _set_item
    value = self._sanitize_column(key, value)
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/frame.py", line 2638, in _sanitize_column
    value = _sanitize_index(value, self.index, copy=False)
  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/series.py", line 2798, in _sanitize_index
    raise ValueError('Length of values does not match length of ' 'index')
ValueError: Length of values does not match length of index