ModelicaMixin: Fix reading of ca.DM nominals

If the nominal from the model was a ca.DM instance, it would raise an
exception when calling np.isnan on it.

For performance reasons we want to avoid naively converting to float
twice, at least for the common case of the nominal being non-symbolic. To
that end, we use a similar resolving check for nominals as was already in
place for e.g seed() and bounds().

src/rtctools/optimization/modelica_mixin.py

@@ -347,10 +347,13 @@ def __nominals(self):
             # If nominal contains parameter symbols, substitute them
             if isinstance(nominal, ca.MX) and not nominal.is_constant():
                 [nominal] = substitute_in_external([nominal], self.__mx['parameters'], parameter_values)
+                if not nominal.is_constant() or np.isnan(float(nominal)):
+                    logger.error('ModelicaMixin: Could not resolve nominal value for {}'.format(sym_name))
+                    continue
 
-            if (not isinstance(nominal, ca.MX) and not np.isnan(nominal)) or nominal.is_constant():
-                nominal = float(nominal)
+            nominal = float(nominal)
 
+            if not np.isnan(nominal):
                 # Take absolute value (nominal sign is meaningless- a nominal is a magnitude)
                 nominal = abs(nominal)