add NotImplementError in __iter__ IterContainer

[arturponinski]

This PR has the objective of suggesting what needs to be implemented in Table dervived classes - related to: https://github.com/petl-developers/petl/issues/483

Changes

1. Added default __iter__ to IterContainer

Checklist

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• [ ] Source Code rules apply:
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes are documented in docs/changes.rst
• [ ] Versioning and history tracking rules apply:
  • [ ] Using atomic commits when possible
  • [ ] Commits are reversible when possible
  • [ ] There is no incomplete changes in the pull request
  • [ ] There is no accidental garbage added in source code
• [ ] Testing rules apply:
  • [x] Tested locally using tox / pytest
  • [x] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [x] Ready to review
  • [ ] Ready to merge

[coveralls]

Pull Request Test Coverage Report for Build 1813574740

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.006%) to 91.865%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
petl/util/base.py	1	2	50.0%
<!--	Total:	1	2	50.0%	-->

Totals
Change from base Build 1813545484:	-0.006%
Covered Lines:	12455
Relevant Lines:	13558

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💛 - Coveralls