Swap time for year keeping subannual resolution

[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

This PR migrates a feature first developed in https://github.com/openENTRANCE/nomenclature to split out the "time" column of an IamDataFrame into a year column (as int) plus keeping the information on the sub-annual resolution as a new extra-column "subannual".

[znicholls]

Very nice

[codecov]

Codecov Report

Merging #557 (52e3e4c) into main (6de47cc) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #557   +/-   ##
=====================================
  Coverage   93.6%   93.6%           
=====================================
  Files         48      50    +2     
  Lines       5242    5277   +35     
=====================================
+ Hits        4909    4944   +35     
  Misses       333     333           

Impacted Files	Coverage Δ
setup.py	0.0% <ø> (ø)
tests/test_core.py	100.0% <ø> (ø)
pyam/core.py	94.2% <100.0%> (-0.2%)	⬇️
pyam/index.py	98.0% <100.0%> (+0.1%)	⬆️
pyam/time.py	100.0% <100.0%> (ø)
tests/test_time.py	100.0% <100.0%> (ø)

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[danielhuppmann]

Thanks for the very swift review, @znicholls!