Describe the game state model

# Modes 2 ways:
## Computer representation
- xarray

## Human representation

```
class GameState:
board_sate : np.array
next_piece : enum
current_piece : enum
score : int
starting_level : int
level : int
stats : dict(enum, int) # optional?
turn : int
current_fps : int

def calculate_fps(current_level)

def current_possible_moves(board_state, current_piece, current_fps)

def find_turn_changes(current_piece, next_piece)
```

Review the proposed computer and human representations in the issue, including the GameState fields and methods. Clarify the intended model and document how the two representations relate; done should be a complete, agreed description of the game state model.

Written by the indexing model from the issue text.