Support mixed 'year' and 'datetime' domain

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR implements refactors the IamDataFrame such that it supports "mixed" time domains (yearly, categorical, datetime), per option 3 of #596.

The PR implements the basic i/o workflow. It does not ensure that all methods support mixed time domain. ToDo:

• [x] Support initialization of an IamDataFrame instance that has year (int) and datetime time domain
• [x] Add a "time domain" attribute
• [x] Support filter (including change of time-column when filtering from mixed to year-only domain)
• [x] Adjust append()
• [x] Adjust concat()
• [x] Support casting to "wide" format via timeseries()

closes #596

[codecov[bot]]

Codecov Report

Merging #598 (948682a) into main (8689a59) will increase coverage by 0.2%. The diff coverage is 99.4%.

@@           Coverage Diff           @@
##            main    #598     +/-   ##
=======================================
+ Coverage   94.3%   94.6%   +0.2%     
=======================================
  Files         54      57      +3     
  Lines       5444    5646    +202     
=======================================
+ Hits        5138    5342    +204     
+ Misses       306     304      -2     

Impacted Files	Coverage Δ
tests/test_feature_rename.py	100.0% <ø> (ø)
pyam/utils.py	90.8% <92.8%> (-1.1%)	:arrow_down:
pyam/core.py	94.9% <98.8%> (+0.3%)	:arrow_up:
pyam/filter.py	100.0% <100.0%> (ø)
pyam/index.py	98.2% <100.0%> (+0.1%)	:arrow_up:
pyam/time.py	98.0% <100.0%> (+1.9%)	:arrow_up:
tests/conftest.py	100.0% <100.0%> (ø)
tests/test_core.py	100.0% <100.0%> (ø)
tests/test_feature_append_concat.py	100.0% <100.0%> (ø)
tests/test_filter.py	100.0% <100.0%> (ø)
... and 5 more

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[danielhuppmann]

Merging per announcement in the Slack workspace.