Pyparam

- Allow default value of other types to be None
- Fix None value conversion at runtime

- Don't use `hasattr` for `__contains__` for Namespace, which allows override of `__getattr__`
- Move help_modifier to params construct
- Update HelpAssembler when command is reused.
- Allow completions for show=True only parameters
- Add kwargs of params to to_dict
- Fix fish completion shell code
- Fix prog update for Params
- Separate defaults of params and param to save size of dumped dict/file
- Add update (`__or__`, `__ior__`) for namespace
- Fix positional/command next to bool from cli
- Fix decendents of ns parameter are hidden from completions if the ns parameter is hidden.

- Add help_modifier argument for `params.parse` to modify help parameters and commands
- Add `params.to_file` to dump `params`
- Allow command reuse
- Add `force` for `params.from_file`, `params.from_dict` to force adding parameters and commands

- Allow parameter reuse
- Add shell completions support
- Add dir parameter type. It is not different as path parameter, but it show different complete candidate for shell completions.

- Fix required list parameter not raise error when on value provided

- Add default to from_arg