Pipeline-integrity

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+ Editions 1991 and 2012 for ASME B31G method

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- Fix asme safe_pressure method zero case.

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+ Localization feature for ASME B31G method explain text

- Fix error in ASME B31G method explain text

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+ Calculation explain facility for ASME B31G method

+ Python2 support

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* Replace pep257 - > pydocstyle in tests.

+ String representation for classes `metal_loss.Item`, `Pipe`, `Context`.

* Named constant 777.

* `Material` class replaced by external `pipeline-material` package.

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+ ASME B31G method