Refactor config.toml - Separate settings to single files

After cleaning the code and the cli, we could switch back, splitting the config file into separated files. This would lead to a cleaner call with less arguments. Now we are calling for example:

$ cod-growth-model ./config/config.toml one_dim

if we separate config.toml in one file per model, you would call:

$ cod-growth-model ./config/one-dim.toml