decision_function: decision_function
In KennethEnevoldsen/SiRToM: SirToM: Simulation in R: Theory of Mind.

k-ToM's decision function, where it calculates its own choice probability based on the updated estimates

1	decision_function(hidden_states, params, player, level, p_matrix)

`hidden_states`	k-ToM's updated estimates
`params`	a list structure containing k-ToM's volatility and behavioural temperature parameters
`player`	k-ToM's player role, i.e. which side of the payoff matrix is used
`level`	k-ToM's sophistication level k
`p_matrix`	a given 2-by-2 payoff matrix