Function place_field_detection 

pub fn place_field_detection(
    binary_train: &[i32],
    positions: &[f64],
    n_bins: usize,
    threshold_std: f64,
    dt: f64,
) -> Vec<(f64, f64)>

Detect place fields as contiguous bins with rate > mean + threshold_std * std. O’Keefe & Dostrovsky 1971.

Returns list of (field_start_pos, field_end_pos).