Add a way to ignore a field, without having to set `init=False`

[lebrice]

Is your feature request related to a problem? Please describe. Would be nice to have a way to tell SimpleParsing to simply ignore a field, (i.e., to not try to parse it via command-line.)

Describe the solution you'd like Something simple, elegant, and easy to implement.

Describe alternatives you've considered

• Adding a ignore=True/cmd=False kwarg to field()
• (bad idea) detecting some flag/specific strings in the field 'docstring'

Additional context This would be very helpful when trying to save/load state to files, for example. Your dataclass might have some command-line arguments as well as some attributes that are used to keep some kind of state. If field(init=False) is used, then the dataclasses.asdict(...) function doesn't return those fields, and they can't be passed to the constructor when creating them from a dictionary (read from a JSON file, for example)

[lebrice]

Fixed, you can pass cmd=False to field() and it will not add any arguments.