planted_solution JSON fix

[jp7745]

We are currently not enforcing this in the schema, so it slipped through until now.

Please integrate requirements and features into the list of instance_data. For example:

"instance_data":[
    {
        "supporting_files": ... 
        "requirements": { }  # one object, not a list.
        "features": {}  # one object, not a list.
    }
]

[jtcantin]

Addressed in jtcantin-problem_instance_updates branch, commit eb8dc1905ea171ebbad00e7e2474f3feb342829f