RFC, ENH: Implement GAMReader and GAMWriter

[oyamad]

Cc: @GautamsGitHub

A couple of proposals:

1. Alternative implementation of conversion from GAM format:

   QuantEcon.py/quantecon/game_theory/game_converters.py

   Lines 130 to 147 in af6038b

   	def _parse(string):
   	tokens = string.split()
   	N = int(tokens.pop(0))
   	nums_actions = tuple(int(tokens.pop(0)) for _ in range(N))
   	payoffs = np.array([str2num(s) for s in tokens])
   	na = np.prod(nums_actions)
   	payoffs2d = payoffs.reshape(N, na)
   	players = [
   	Player(
   	payoffs2d[i, :].reshape(nums_actions, order='F').transpose(
   	list(range(i, N)) + list(range(i))
   	)
   	) for i in range(N)
   	]
   	return NormalFormGame(players)

2. Rename qe_nfg_from_gam_file to from_gam
3. Add GAMWriter and to_gam

[coveralls]

coverage: 93.111% (-0.004%) from 93.115%
when pulling af6038b on gam_reader
into 66a4451 on main.

[mmcky]

thanks @oyamad

Are these internal functions? Should we add any deprecation notifications for the old function names?

[oyamad]

@mmcky Yes and no: On the current main, qe_nfg_from_gam_file is an internal function (i.e., not exported); and no version has been released since this function was implemented (so no need for deprecation notification).

[mmcky]

thanks @oyamad. It looks like you're waiting for comments from @GautamsGitHub -- so feel free to merge when you're ready @oyamad.

I will keep an eye out and organise a new release once merged.

[oyamad]

Let's merge this PR.