MatrixXr serialisation now compatible with PyYaml and more consistent with Vectors.

Modules/LemmaCore/include/helper.h

     node["cols"] = rhs.cols();
     for (int ir=0; ir<rhs.rows(); ++ir) {
         for (int ic=0; ic<rhs.cols(); ++ic) {
-            node[ir][ic] = rhs(ir,ic);
+            node["data"][ir][ic] = rhs(ir,ic);
         }
-        node[ir].SetStyle(YAML::EmitterStyle::Flow);
+        node["data"][ir].SetStyle(YAML::EmitterStyle::Flow);
     }
     //node.SetStyle(YAML::EmitterStyle::Block);
     node.SetTag( "MatrixXr" );
     rhs.resize(nr, nc);
     for (int ir=0; ir<nr; ++ir) {
         int ic=0;
-        for(YAML::const_iterator it=node[ir].begin(); it!=node[ir].end(); ++it) {
+        for(YAML::const_iterator it=node["data"][ir].begin(); it!=node["data"][ir].end(); ++it) {
             rhs(ir,ic) = it->as<Lemma::Real>();
             ++ic;
         }