Scriptengine

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Features
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- Fix 71: Add Jinja2 filters 'exists' and 'path_join' (thanks jmrgonza)
- Add documentation for SE filters for Jinja2

Fixes
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- Fix 66: Some fixes and completions to documentation (thanks mandresm)

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Fixes
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- Fix 68: Saving stderr/stdout from base.command fails in jobs
(thanks jmrgonza for reporting)

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Fixes
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- Fix 64: deepdiff 6.2.1 breaks base.context with rrules
(thanks madnresm and JanStreffing for reporting)

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Fixes
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- Fix 59: loop on strings tuple not working as expected
- Fix 61: base.getenv not returning context update

Internal changes
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- Some refactoring in job loop code
- Some refactoring in Include task code

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Fixes
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- Fix 58: SE installation error

===================

Fixes
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- Fix 8: Loops with empty or undefined specs
- Fix 54: base.include task does not update context
- Fix typo in Include task argument (ignore_not_found)
- Fix 55: base.include messes up the context
- Fix 56: Wrong loop variable collision warnings
- Fix 57: Context append to list may drop items

Internal changes
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- Introduce Context and ContextUpdate classes