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Description of Changes

The default reducer puts out the evaluator and a bunch of other C++ extension types that pickle then chokes on. Side step the problem here by providing a reducer that just makes unpickling equivalent to a new instantiation. The only difficulty here is that basis_set stays a file name if initialized like that and that PyACECalculator.init expects it as a positional argument.
Avoid the filename by dumping the whole bbasis config, so that unpickling can happen on other machines or in different locations.

Checklist

• Code is well-documented.
• All tests have been run and passed.
• Relevant documentation has been updated if necessary.

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• The code submitted in this pull request will be distributed under the Academic Software License (free for academic non-commercial use, not free for commercial use), see LICENSE.md for more details.
• The copyright for the code, including the submitted code, remains with Ruhr University Bochum. Ruhr University Bochum retains the right to transfer or modify the copyright.

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