Explicitly convert model config options to float

This seems to be necessary because the yaml parser doesn't understand
numpy scalars.

polaris/ocean/tasks/manufactured_solution/forward.py

@@ -76,9 +76,9 @@ def dynamic_model_config(self, at_setup):
 
         exact_solution = ExactSolution(self.config)
         options = {'config_manufactured_solution_amplitude':
-                   exact_solution.eta0,
+                   float(exact_solution.eta0),
                    'config_manufactured_solution_wavelength_x':
-                   exact_solution.lambda_x,
+                   float(exact_solution.lambda_x),
                    'config_manufactured_solution_wavelength_y':
-                   exact_solution.lambda_y}
+                   float(exact_solution.lambda_y)}
         self.add_model_config_options(options)