Validate parsing_demo.py

[epogrebnyak]

Ideas/intents: parsing_overview.md

Todo in parsing_demo.py:

• review algorithm, comment where surprises
• suggestions for better naming of vars, funcs
• other comments

[epogrebnyak]

Todo: document code and add doctest as in yield_rows_as_dicts()

[epogrebnyak]

Note: possible wrapper class Datapoints(csv_content, parsing_instruction).emit(freq)

[epogrebnyak]

reviewed parsing_demo.py saved at new branch https://github.com/epogrebnyak/data-rosstat-kep/blob/doc_parsing_demo/doc/parsing_demo.py and integrated to https://github.com/epogrebnyak/data-rosstat-kep/blob/kep2/kep2/datapoints.py