Fix Problem.to_files(model_file=...) (#204)

* Fix Problem.to_files(model_file=...)

model_file parameter was not properly handled, resulting in missing model files in the created yaml file.

* typo

---------

Co-authored-by: dilpath <dilan.private+github@outlook.com>

Author: dweindl

petab/problem.py

@@ -411,7 +411,7 @@ def to_files_generic(
             if not isinstance(self.model, SbmlModel):
                 raise NotImplementedError("Saving non-SBML models is "
                                           "currently not supported.")
-            filenames['sbml_file'] = 'model.xml'
+            filenames['model_file'] = 'model.xml'
 
         filenames['yaml_file'] = 'problem.yaml'
 
@@ -542,7 +542,7 @@ def error(name: str) -> ValueError:
 
         if yaml_file:
             yaml.create_problem_yaml(
-                sbml_files=sbml_file,
+                sbml_files=model_file,
                 condition_files=condition_file,
                 measurement_files=measurement_file,
                 parameter_file=parameter_file,