Function compute_group_connectivity 

pub fn compute_group_connectivity<P: TrxScalar>(
    trx: &TrxFile<P>,
    group_names: &[&str],
    measure: ConnectivityMeasure,
    dps_weight_name: Option<&str>,
) -> Result<Vec<f64>>

Compute a pairwise connectivity matrix between groups.

Returns a packed upper-triangle vector of size n*(n+1)/2 where n = group_names.len(). Entry (i,j) with i <= j is at index i*n - i*(i+1)/2 + j.

Each streamline is assigned to a group if its index appears in that group’s member list. A streamline connecting groups i and j increments the (min(i,j), max(i,j)) entry.