Fix PySBModel.get_parameter_value (#203)

Incorrectly returned the parameter name, instead of its value.

Author: dweindl

petab/models/pysb_model.py

@@ -83,7 +83,7 @@ def get_parameter_ids(self) -> Iterable[str]:
 
     def get_parameter_value(self, id_: str) -> float:
         try:
-            return self.model.parameters[id_].name
+            return self.model.parameters[id_].value
         except KeyError as e:
             raise ValueError(f"Parameter {id_} does not exist.") from e