Add an IamDataFrame.series attribute to get internal _data

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

I'm accessing the _data object for a lot of computations, but that yields warnings because of access to an internal attribute. So this PR adds a utility method.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.7%. Comparing base (144a9eb) to head (7c0f55c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #989   +/-   ##
=====================================
  Coverage   94.7%   94.7%           
=====================================
  Files         70      70           
  Lines       6717    6726    +9     
=====================================
+ Hits        6365    6374    +9     
  Misses       352     352           

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