Refactor File Types

[wpm]

Make Themis file types subclass pandas.DataFrame. Put the code to load them via argparse in a static method. Change the output_format static method to __str__. Make __repr__ a head of the table along with the number of rows it contains. Move from_csv, to_csv, and print_csv functionality into these objects. Make column header names static variables inside the objects.

Pull these objects out into a separate source file and write documentation for each of them. This will make the code easier to read and more self-documenting.

No functionality change.