Refactor IIASA-connection-API and rework all related tests

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Description of PR

This PR harmonizes the behavior of the iiasa.Connection class functions, prepares for a refactoring of the IIASA-backend API, and reworks the tests to use a dedicated test database instance (making the tests more precise by comparing full IamDataFrame instance rather than just length of returned objects).

Changes In detail:

• all iiasa-related tests are directed to a dedicated integration-test instance that has only 4 scenario objects based on the default objects used conftest.py, closes Add better documentation and testing of objects returned IIASA-db connection #303
• read_iiasa() and query() return an IamDataFrame instead of a pandas object
  (now including meta-indicators by default), closes Return-type of iiasa.Connection.query() not IamDataFrame #378
• deprecate scenario_list() for an overview of all scenarios in favour of index()
• deprecate available_metadata() in favour of meta_columns (attribute)
• deprecate metadata() in favour of meta()

[codecov]

Codecov Report

Merging #413 into master will increase coverage by 0.10%.
The diff coverage is 92.07%.

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
+ Coverage   93.26%   93.37%   +0.10%     
==========================================
  Files          35       35              
  Lines        4129     4149      +20     
==========================================
+ Hits         3851     3874      +23     
+ Misses        278      275       -3     

Impacted Files	Coverage Δ
pyam/iiasa.py	88.12% <91.30%> (-0.24%)	⬇️
tests/test_iiasa.py	91.66% <92.04%> (+4.37%)	⬆️
pyam/testing.py	75.00% <100.00%> (ø)
tests/conftest.py	97.33% <100.00%> (+0.23%)	⬆️

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[danielhuppmann]

@stickler-ci is incorrectly reporting linting errors (they have been fixed)

[danielhuppmann]

@znicholls @byersiiasa @Rlamboll @gidden, FYI - this PR will make the integration with the IIASA-database-API more stable (because of more and more explicit tests), but it also adds one important change: the functions read_iiasa() and query() directly return an IamDataFrame. Other functions are marked as deprecated (including a log message), but will continue to work for at least one more release.

Apologies for the inconvenience, but I didn't find a practical way to avoid the immediate pain on the return type. Please ping any users that you know to be using this feature.

[znicholls]

Found one typo, otherwise my quick glance didn't see anything obviously problematic. Thanks for the heads up @danielhuppmann !!