Seq2pat

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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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Major:
- Add one-hot encodings, by mining patterns locally (csp_local.py) or globally
(csp_global.py) in each sequence
- Add Dichotomic Pattern Mining notebook, running on sample data
- Clean up C++ vector variables datab_MDD, mdd_q from calling Build_MDD
- Clean up vector variable attr in node_mdd.hpp
- Clean up vector variables indic_vec, result from calling Freq_miner

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