For the MetaData, we need to use a DataFrame to collect data for each model. It's to make it easier for users to display the provided models in this package just by typing :

using OptimalControlProblems
data = OptimalControlProblems.metadata

For each model, I suggest we provide the following information:
- name: problem name
- nh: default number of discretization points
- nvar: number of variables
- ncon: number of general constraints
- minimize: true if optimize == minimize