Group update

Changes

• Remove World (and NodesCreator)
  • Change way of nodes and networks creation
• Add support for new recommend way of importing TestTools (with import test_tools as tt)
• Add docstrings and type annotations for public method of nodes and networks
  • Ensure public API quality with additional Pylint checks run at CI:
    • Every public functions must contain docstring,
    • Every parameter must be described,
    • Every parameter must have type annotation.
• Flatten generated directories structure. Before nodes' and wallets' data were generated in network's directory. Now network's directory is not generated at all. (It is needed to allow in the future to change nodes' networks).
• Make CleanupPolicy configurable locally with syntax tt.cleanup_policy.set_default(tt.constants.CleanupPolicy.DO_NOT_REMOVE_FILES)
• Update documentation to match introduced changes.

Internal changes

• Separate public (user's) API from internal API in node's and network's
  • Add helpers to switch between handles (user interface) and implementations (internal interface)
• Reorder packages and modules:
  • Rename private directory to __private
  • Hide following modules: account, cleanup_policy, communication, constants, exceptions, network, node_config, paths_to_executables, wallet
• Nodes are scoped objects now
• Remove ChildrenNames class (is replaced by Names)
• Workaround problem reported by Pylint about missing list's methods in config entries which are lists
• Many refactors