Pytest-instrument

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- Refactors `setLoggerClass` implementation to a `Filter`-based one to avoid side effects of `setLoggerClass`.

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- Next to `json` the `--instrument` parameter now also accepts `log` to send output to a regular log file.

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- The `--instrument` parameter now requires value, e.g. `--instrument=json` (only one supported currently).
- Next to the "instr.report" logger there is now an "instr.log" logger you can use in your tests to emit records to the same file.
- The `.log` output file is no longer first written as a pickle, then converted to a json array. It is written immediately and contains json objects.
- Tests were added and existing ones were refactored.

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- initial version