RFC: Refactor game_converters.py (#826)

* ENH: Add `GAMPayoffVector`

* RFC: Refactor `GAMWriter._dump`

- Use `io.StringIO()`
- Fine tune `np.array2string`
- Add tests for `to_gam`

* RFC: Refactor `GAMReader._parse`

Also add `from_gam_string` and `from_gam_url`

* TEST: Add tests in test_game_converters.py

* DOC: Edit docstring in game_converters.py

* Remove `random_game`

* Remove backticks

Author: oyamad

quantecon/game_theory/__init__.py

@@ -24,4 +24,6 @@
 from .logitdyn import LogitDynamics
 from .polymatrix_game import PolymatrixGame
 from .howson_lcp import polym_lcp_solver
-from .game_converters import GAMReader, GAMWriter, from_gam, to_gam
+from .game_converters import (
+    GAMReader, GAMWriter, from_gam, from_gam_string, from_gam_url, to_gam
+)