Rename "namespace" in add_commands()

[neithere]

Problem: the "namespace" term is ambiguous in the context of argparse / argh.

• The argparse.Namespace class represents the result of parsing (returned by ArgumentParser.parse()). It could have been a dictionary of attributes and their values; could have been named ParsingResult.
• The namespace argument in add_commands() is a str, the name of the, well, namespace for commands in a subparser; a subparser label. The run in prog.py web run.

Options:

• ArgumentParser.add_subparsers() mentions "command" and "sub-parser group".
• "Group" is associated with argument groups (simply groups of arguments lumped together when displaying help) and mutually exclusive groups.

Tasks:

• decide on the name
• introduce it everywhere in the code
• document the usage
• deprecate the old name
• make sure the deprecation is documented, warnings are tested, removal is scheduled for a future version (e.g. 0.31)

[neithere]

Will be released in v.0.29.