Scriptengine

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Features
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- Fix 41: Improved stdout handling for Command task
(changes semantics of the 'stdout' argument)
- Fix 42: Handle error messages in Command task


Fixes
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- Fix 37: Missing required argument: logging error

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Features
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- Fix 40: Handling environment variables
This adds base.setenv and changes the syntax for base.getenv!

Internal changes
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- Major simplification of base.context code by using Task.getarg()

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Fixes
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- Fix 39 (again): use shutil.move for base.move task

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Fixes
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- Fix 39: shutil does not always accept path-like objects

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Fixes
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- Fix 38: base.copy handles missing files incorrectly

Features
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- base.copy task supports 'ignore_not_found'

Internal changes
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- Base file tasks use pathlib instead of os module

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Features
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- 36: Add 'basename' and 'dirname' Jinja2 filters