Remove ixmp as optional dependency

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR removes the ixmp packages as an optional dependency. The feature to read IAMC-style scenario data directly from an ixmp database instance was untested, does not support meta-indicators and, as far as I know, is not used by anyone.

The feature to query data from an IIASA Scenario Explorer database (pyam.read_iiasa()) is not affected by this PR!

The PR prepares for the release of ixmp 4.0.0, based on a discussion with @meksor and @phackstock today.

@gidden @OFR-IIASA @behnam-zakeri @byersiiasa, please shout if you are using this feature or know of anyone who does...

[codecov[bot]]

Codecov Report

Merging #688 (4adceb3) into main (d104630) will increase coverage by 0.1%. The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #688     +/-   ##
=======================================
+ Coverage   94.7%   94.8%   +0.1%     
=======================================
  Files         59      58      -1     
  Lines       5836    5814     -22     
=======================================
- Hits        5529    5517     -12     
+ Misses       307     297     -10     

Impacted Files	Coverage Δ
pyam/__init__.py	67.5% <ø> (-0.9%)	:arrow_down:
pyam/core.py	94.8% <ø> (+0.2%)	:arrow_up:

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[danielhuppmann]

Will merge in one week (August 26) if I do not hear any objections...