ENH: Add state_values to DiscreteDP

[HG-Cheng]

This PR partially addresses #248 by adding state_values support to
DiscreteDP.

Changes:

• Add an optional state_values argument to DiscreteDP.
• Validate state_values consistently with MarkovChain.
• Preserve state_values when converting between product and
  state-action-pair formulations.
• Pass state_values to the MarkovChain returned by controlled_mc.
• Add tests for storage, validation, propagation, and representation
  conversion.

action_values and num_actions are intentionally left out of this PR,
following the review discussion.

[coveralls]

coverage: 92.722% (+0.02%) from 92.706% — HG-Cheng:feature-ddp-values into QuantEcon:main

[oyamad]

@HG-Cheng Thank you for the contribution!

Main comments

1. I propose that we only add state_values and remove action_values (and num_actions) in this PR.

   Please see Adding State-Action Pair Support QuantEcon.jl#94 (comment):
   We are implicit which of the two approaches in that comment we are following (although I believe approach 1 is the legitimate approach). We need a discussion to decide on which approach to pick (or even whether to pick one explicitly). I propose not to discuss it here, focusing only on adding state_values in this PR.

2. Please pass state_values=state_values to MarkovChain in controlled_mc: https://github.com/HG-Cheng/QuantEcon.py/blob/fff5405ae7d4f66df60bb2ae39a943504be19c59/quantecon/markov/ddp.py#L1004
   This is where adding state_values is most useful.

Other comments

Please do not include changes in the files .vscode/settings.json and _matrix_eqn.py in this PR.

[HG-Cheng]

"Hi @oyamad, thanks for the detailed review and guidance!

I completely understand. I will remove action_values and num_actions to keep this PR focused on state_values only, and we can leave the state-action pair discussion for the future.

I will pass state_values=state_values to MarkovChain in controlled_mc as suggested.

Apologies for the accidental inclusion of .vscode/settings.json and _matrix_eqn.py! They leaked in from my local workspace and another task. I will remove them from this branch right away.

I'll push the updated commits shortly!"

[HG-Cheng]

Thanks again for the review. I've updated the PR to keep it focused on
state_values only, removed the unrelated changes, and propagated
state_values to the controlled MarkovChain.

I also synced the branch with the latest main. The full local test suite passes locally (605 passed).