Release candidate 1.6

[danielhuppmann]

Description of PR

This PR adds the highlights for the new release and does some minor docstrings-clean-up of the new lazy_ready_iiasa() function.

[codecov]

Codecov Report

Merging #705 (c99d1fa) into main (d7d9b45) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

❗ Current head c99d1fa differs from pull request most recent head 3ea957e. Consider uploading reports for the commit 3ea957e to get more accurate results

@@           Coverage Diff           @@
##            main    #705     +/-   ##
=======================================
- Coverage   94.8%   94.8%   -0.1%     
=======================================
  Files         58      58             
  Lines       5899    5894      -5     
=======================================
- Hits        5598    5593      -5     
  Misses       301     301             

Impacted Files	Coverage Δ
tests/test_iiasa.py	97.8% <ø> (-0.1%)	⬇️
pyam/iiasa.py	86.7% <100.0%> (ø)
pyam/utils.py	91.6% <100.0%> (-0.1%)	⬇️

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[danielhuppmann]

Published on test-pypi successfully, see https://test.pypi.org/project/pyam-iamc/1.6.0rc1/