Add a learning rate feature

[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 adds a method to easily compute the implicit learning rates from "experience" (e.g., cumulative installed capacity) and "performance" (e.g., specific investment) cost from any IamDataFrame with suitable variables.

The new feature is implemented together with a new compute module/accessor to separate out the more advanced timeseries-data-methods from the main class.

[codecov]

Codecov Report

Merging #602 (c5c8bd3) into main (27d0fac) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #602   +/-   ##
=====================================
  Coverage   94.2%   94.2%           
=====================================
  Files         50      52    +2     
  Lines       5361    5415   +54     
=====================================
+ Hits        5052    5106   +54     
  Misses       309     309           

Impacted Files	Coverage Δ
pyam/compute.py	100.0% <100.0%> (ø)
pyam/core.py	94.4% <100.0%> (+<0.1%)	⬆️
pyam/utils.py	91.8% <100.0%> (+<0.1%)	⬆️
tests/test_feature_learning_rate.py	100.0% <100.0%> (ø)

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