Add the outer ld_matrix python code

Overall, the goal was to mirror other sample set stat design patterns,
with the exception that we dispatch each low level stat matrix from the
ld_matrix function.

We also fix some behavior uncovered in testing:

* Ensure the D_prime statistic does not divide by zero. (opened #2907 to
  investigate if this something we actually want to do).
* Look up node from sample_index_map when finding samples under mutation
  nodes.
* Do not error out if no sites are specified, instead return an array
  with (0, 0) dimensions. Remove associated test, add one for early
  return.

Exhaustive test coverage was added, but more could be done to validate
the results from these tests:

Add LD matrix tests, implementing the rest of the summary functions.
Mirror the ld_matrix behavior. More thorough testing of various data
partitions. Compare to LD calculator, and use all of the example tree
sequences from test_highlevel.

c/tests/test_stats.c

@@ -2583,9 +2583,10 @@ test_two_locus_stat_input_errors(void)
     row_sites[0] = 0;
     row_sites[1] = 1;
 
+    // Not an error condition, but we want to record this behavior
     ret = tsk_treeseq_r2(&ts, num_sample_sets, sample_set_sizes, sample_sets, 0, NULL, 0,
         NULL, 0, result);
-    CU_ASSERT_EQUAL_FATAL(ret, TSK_ERR_BAD_SITE_POSITION);
+    CU_ASSERT_EQUAL_FATAL(ret, 0);
 
     tsk_treeseq_free(&ts);
     tsk_safe_free(row_sites);

c/tskit/trees.c

@@ -2471,8 +2471,8 @@ get_mutation_samples(const tsk_treeseq_t *ts, const tsk_id_t *sites, tsk_size_t
             for (n = 0; n < num_nodes; n++) {
                 node = nodes[n];
                 if (flags[node] & TSK_NODE_IS_SAMPLE) {
-                    tsk_bit_array_add_bit(
-                        &mut_samples_row, (tsk_bit_array_value_t) node);
+                    tsk_bit_array_add_bit(&mut_samples_row,
+                        (tsk_bit_array_value_t) ts->sample_index_map[node]);
                 }
             }
         }
@@ -2612,9 +2612,8 @@ check_sites(const tsk_id_t *sites, tsk_size_t num_sites, tsk_size_t num_site_row
     int ret = 0;
     tsk_size_t i;
 
-    if (sites == NULL || num_sites == 0) {
-        ret = TSK_ERR_BAD_SITE_POSITION; // TODO: error should be no sites?
-        goto out;
+    if (num_sites == 0) {
+        return ret; // No need to verify sites if there aren't any
     }
 
     for (i = 0; i < num_sites - 1; i++) {
@@ -3638,9 +3637,10 @@ D_prime_summary_func(tsk_size_t state_dim, const double *state,
         double p_B = p_AB + p_aB;
 
         double D = p_AB - (p_A * p_B);
-        if (D >= 0) {
+        result[j] = 0;
+        if (D > 0) {
             result[j] = D / TSK_MIN(p_A * (1 - p_B), (1 - p_A) * p_B);
-        } else {
+        } else if (D < 0) {
             result[j] = D / TSK_MIN(p_A * p_B, (1 - p_A) * (1 - p_B));
         }
     }