Timelink

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* fix bug with to_kleio() in models (bad identation)

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* Fix bug with export_to_kleio()
* Several minor bug fixes

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* Adds new eattributes views to easily access line, level, groupname of attributes
* pandas.entities_with_attribute returns attribute type,line,level and obs.
* Adds reference requirements.txt file to notebook tests
* Adds TimelinkDatabase.export_as_kleio(ids,filename

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Fixes regression in 1.1.5.

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* Comments and original wording are now stored
in the "obs" field of entities, preceeded by
"extra_info:".

* If obs field contained explicit observations,
these are preserved, and "extra_info:" is appended.

* A new method in the Entity class, get_extra_info()
fetches the extra info as a dictionnary.

* Partially implements issue 9.

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Skips long imports in Travis CI