Add a method to visualize the policy values of multiple policies

[Kurorororo]

I implement a method to visualize the policy values of multiple policies.

When you do the following in examples/quickstart/synthetic.ipynb,

ope.visualize_off_policy_estimates_of_multiple_policies(
    policy_name_list=["IPW LR", "IPW RF", "Random"],
    action_dist_list=[action_dist_ipw_lr, action_dist_ipw_rf, action_dist_random],
    estimated_rewards_by_reg_model=estimated_rewards_by_reg_model,
    n_bootstrap_samples=100, # number of resampling performed in the bootstrap procedure
    random_state=12345,
)

[usaito]

@Kurorororo Thanks!