Allow grouping of argparse fields according to subclassing

[reuben]

When using inheritance to extend config definitions the resulting ArgumentParser has all fields flattened out. It would be nice to group fields by class and allow some control over ordering.

[erogol]

What do you mean by grouping ?

Just for argparse or in the Coqpit class too?

How would you to_json it if we group by class?

[reuben]

Just for argparse, which lets you have argument groups with titles/descriptions. JSON grouping is already handled with nested configs.