Different approach to unstack in `timeseries()`

[danielhuppmann]

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Description of PR

I ran into a MemoryError yesterday when writing a large IamDataFrame to file so I tried to implement a more performant approach for pvioting to the wide IAMC format.

[danielhuppmann]

Re questions by @phackstock:

I did not yet run any profiling and this change will (unfortunately) not change the performance issues when reading data.

[phackstock]

Ah ok thanks for the clarifications @danielhuppmann. Still very good to improve data writing performance.

[codecov[bot]]

Codecov Report

Merging #720 (840d738) into main (a39fdca) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #720   +/-   ##
=====================================
  Coverage   95.0%   95.0%           
=====================================
  Files         59      59           
  Lines       5998    6004    +6     
=====================================
+ Hits        5702    5708    +6     
  Misses       296     296           

Impacted Files	Coverage Δ
pyam/core.py	95.4% <100.0%> (+<0.1%)	:arrow_up:
tests/test_core.py	100.0% <100.0%> (ø)
tests/test_time.py	100.0% <100.0%> (ø)

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[danielhuppmann]

Performance tests indicate that this was a very bad idea... But I pulled out some minor improvements into #723