Show independent Python models for two instances of a component

[ru551n]

Adds accumulator_model, a component with a stateful Python model, to the embedded_python example. The new test case Test independent models of two instances instantiates it twice; each instance makes a session with new_session(accumulator_model'instance_name), executes the same accumulator_model.py into it with exec_file after test_runner_setup, and passes the session to every call. The two instances get different inputs, and the test checks that each one keeps its own total.

The user guide gets a new section, Independent models of component instances, under Sessions. It explains why repeated instances need a session each, why a fixed name doesn't separate them and how 'instance_name does, and that NVC, GHDL or Questa is needed. It links to the runnable example and shows the class-based alternative (one object per session, state on self). The caveats now also say that state in the globals of an imported module isn't isolated.

The session is made in the statement part of the process. GHDL (7.0.0-dev) leaves the instance labels out of 'instance_name (and 'path_name) when it is evaluated in a declaration: both instances got :tb_example(tb)accumulator_model(python):, so they shared one session. The docs mention this.

Fixes #8

Testing

• examples/embedded_python/run.py --without-attributes .optional_deps --without-attributes .expected_failure: 18 of 18 pass on NVC and on GHDL.
• tests/run.py: 106 of 106 pass on NVC and on GHDL. pytest tests/test_python_bridge.py: 85 passed.
• Checked that the test catches sharing: with a fixed session name, it fails (Got 12. Expected 3.).
• Ran the class-based doc snippet in a scratch testbench with two instances: passes on NVC and GHDL.
• sphinx-build of the docs: no warnings.

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