[TPX] Add factory function for Substance from species name

This function had been defined but not declared.

include/cantera/tpx/utils.h

@@ -7,6 +7,22 @@
 namespace tpx
 {
 Substance* GetSub(int isub);
+
+//! Create a new Substance object corresponding to the specified name.
+/*
+ * Currently valid substances are:
+ *
+ * - water
+ * - nitrogen
+ * - methane
+ * - hydrogen
+ * - oxygen
+ * - hfc134a
+ * - carbondioxide
+ * - heptane
+ */
+Substance* newSubstance(const std::string& name);
+
 }
 
 #endif

src/tpx/utils.cpp

@@ -18,7 +18,7 @@
 
 namespace tpx
 {
-Substance* GetSubstanceByName(std::string name)
+Substance* newSubstance(const std::string& name)
 {
     std::string lcname = boost::algorithm::to_lower_copy(name);
     if (lcname == "water") {
@@ -40,7 +40,8 @@ Substance* GetSubstanceByName(std::string name)
     } else if (lcname == "heptane") {
         return new Heptane;
     } else {
-        return 0;
+        throw Cantera::CanteraError("tpx::newSubstance", "No Substance"
+            " definition known for '{}'.", name);
     }
 }