ValueOrRange Serialize Issue

[bjohnson5]

Describe the bug
Serializing a ValueOrRange type to json turns the object into a string. Then when deserializing the sim.json file you get this error:

data did not match any variant of untagged enum ValueOrRange

This is really only an issue if you are using simln as a library and creating simulations directly in rust (not loading sim.json files)

To Reproduce

• Use sim-lib to create a simulation with a list of ActivityParser types
• Use serde_json::to_string to serialize the list to json
• Write the json data to a file so that it can be used as a sim.json for in a later simulation
• Try to load the saved sim.json file and get the error

I believe this is due to the custom serializer that simply uses serialize_str to serialize the ValueOrRange