'to_nexus()' emits basically compliant Nexus

Author: jeetsukumaran

python/tskit/trees.py

@@ -5149,8 +5149,9 @@ def to_nexus(self, precision=14):
 
         s = "#NEXUS\n"
         s += "BEGIN TAXA;\n"
+        s += "DIMENSIONS NTAX={};".format(len(node_labels))
         s += "TAXLABELS "
-        s += ",".join(node_labels[node.id] for node in self.nodes()) + ";\n"
+        s += " ".join(node_labels[node.id] for node in self.nodes()) + ";\n"
         s += "END;\n"
 
         s += "BEGIN TREES;\n"