Massive rewrite of the CLI and lots of refactoring work.
- Changed the concept of CLI context and workspaces (see README).
- Moved 'pycdstar3.client' module to 'pycdstar3.api'.
- Moved data classes from 'pycdstar3.api' to 'pycdstar3.model'.
- Introduced optional high-level API handle classes for remote resources.
TODO: Tests