ENH: Revise Python API for loading/saving games

[tturocy]

Game objects currently just have generic .read_game and .write_game operations, where .write_game takes a format parameter to determine the output format, while .read_game tries various formats until one is successful.

Looking at e.g. pandas, it seems it would be better to have functions that explicitly read and write the different supported formats.

[tturocy]

As an initial concrete proposal, we suggest we'll follow the conventions that pandas uses in its I/O functions. Specifically, one can look at read_csv (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.read_csv.html) and to_csv (https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.to_csv.html).

Specifically, we will have the following:

• The functions for reading files will be module-level in pygambit. (The current functions are all part of the Game class.)
• When reading, the first argument should be a string, path object, or file-like object, just like read_* in pandas.
• When writing, the first argument is optional, and can be a string, path object, or file-like object, or null. If null, the function returns the serialised string representation.

We will have the following functions (please add a note if there are others that are missing)

• Reading
  • read_efg - reads an .efg file format
  • read_nfg - reads an .nfg file format
  • read_gbt - reads a .gbt file (XML files produced by the GUI)
  • read_agg - reads an action-graph games file format
• Writing
  • Game.to_efg - writes an .efg file
  • Game.to_nfg - writes an .nfg file
  • Game.to_gbt - writes a .gbt file
  • Game.to_html - writes out HTML tables
  • Game.to_latex - currently the sgame format supported by Game.write.
  • (there is not at present support for writing out AGG files so no need to have this as yet)

The function Game.parse_game will be deprecated, as its functionality can be replicated by wrapping such a string in an io.StringIO object and then passing it to the appropriate read_* function as above, which functions will now accept file-like objects.