Optimize default parameter grid

[nanxstats]

Previous grid:

• n_iterations = c(10, 50, 100, 200, 500, 1000)
• max_depth = c(2, 3, 4, 5)
• learning_rate = c(0.001, 0.01, 0.02, 0.05, 0.1)

Optimized grid:

• n_iterations = c(100, 200, 500, 1000)
• max_depth = c(3, 5, 7, 9)
• learning_rate = c(0.01, 0.05, 0.1, 0.2)

These values follow the common ranges of the three parameters, and provide a balance between model complexity, risk of overfitting, and training time.