Accept X as alias for atom symbol Nn

src/atom_type.cpp

@@ -1029,6 +1029,9 @@ atom_type_traits::atom_type_traits(const std::string& symbol)
 		}
 		}
 	}
 	}
 
 
+	if (symbol == "X")
+		m_info = &data::kKnownAtoms[0];
+	
 	if (m_info == nullptr)
 	if (m_info == nullptr)
 		throw std::invalid_argument("Not a known element: " + symbol);
 		throw std::invalid_argument("Not a known element: " + symbol);
 }
 }