Brier Score and Reliability

The Brier score is the mean squared error of probability forecasts: $\frac1n\sum (p_i - y_i)^2$.

Implement:

• brier(y, p)
• reliability_table(y, p, bins=10): split [0, 1] into equal-width bins (a probability of exactly 1.0 goes in the last bin) and return a list of (mean_predicted, observed_rate, count) for non-empty bins in order