Function set_value_f64 

pub fn set_value_f64(
    a: &mut SparseMatrix<f64>,
    row_idx: i32,
    col_idx: i32,
    val: f64,
) -> Result<()>

Set a single value at (row_idx, col_idx) in a CSR matrix handle. Element must already exist in the sparsity pattern. (f64)