Scriptengine

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Fixes
-----
- 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
----------------
- Introduce Context and ContextUpdate classes

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Fixes
-----
- Fix 33: When clause with empty/undefined expression

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

Fixes
-----
- Fix 53: Looping over list of dicts doesn't work in particular case

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

Fixes
-----
- Fix 52: Error when Python 3.6 deepcopy copies context

Features
--------
- Fix 47: Loop over dictionaries
- Introduce context["se"]["tasks"]["last_result"]
- Github workflow for automatic testing

Internal changes
----------------
- Task.run() never changes context, returns dict or deepdiff.Delta
for context updates
- Major changes in Job implementation
- SimpleScriptEngine relies on Job class for run()

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Fixes
-----
- Fix 46: Add optional `executable` argument to base.template
- Fix 48: stdout/stderr handling in base.command

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

Fixes
-----
- Fix 44: Command task log not correct
- Fix 45: Command task in 0.10 doesn't work with Python <3.7