Seq2pat

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Major:
- Update CI test environment to Python 3.8, 3.9, 3.10, 3.11 and drop support for Python 3.7
- Update installation requirement to Python 3.8+
- Update CI test environment to drop support for Windows-latest tests

Minor:
- New section in README to explain max_span and batch_size parameters for mining large sequence databases.

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Major:
- Enable running Seq2Pat on batches of sequences in parallel
- Add a usage example notebook to show how batch processing is called
- Enrich documentations and comments relevant to batch processing and a few tests
- Explicitly require that input integer sequences only contain positive integers

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Minor:
- Fix a compiling issue in using Visual Studio C++ compiler

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Major:
- Add a built-in maximum span constraint on items' indices to simplify and speed up usages
- A fix to use original gap value instead of abs gap value in Check_gap() in build_mdd.cpp
- Update unit tests where the built-in constraint may change the mining results

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Minor:
- Refactor encodings generation component as Pat2Feat

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Minor:
- Set the default behavior of drop_pattern_frequency in encoding to be True

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