[BUG]: Cannot serialize template model that has datetime in annotations

[liunelson]

I created a TemplateModel with annotations. When the annotation fields are time_start, time_end are given datetime strings, the values are converted to datetime objects and the function template_model_to_petrinet_json(...) does not know to convert them back to strings before JSON serialization.

model = TemplateModel(
    ..., 
    annotations = Annotations(
        ...,
        time_start = "2020-03-01T00:00:00",
        time_end = "2020-08-01T00:00:00"
    )
)

with open("./model.json", "w") as f:
    json.dump(template_model_to_petrinet_json(model), f, indent = 4)