Migrate `compute_bias()` to `compute` module

[danielhuppmann]

Please confirm that this PR has done the following:

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

Description of PR

To remove clutter from the core module, this PR moves the method compute_bias() to compute module (with a deprecation-warning until the next release).

[codecov[bot]]

Codecov Report

Merging #702 (b23f78c) into main (fde3690) will decrease coverage by 0.0%. The diff coverage is 75.0%.

@@           Coverage Diff           @@
##            main    #702     +/-   ##
=======================================
- Coverage   94.7%   94.7%   -0.1%     
=======================================
  Files         58      58             
  Lines       5853    5855      +2     
=======================================
  Hits        5547    5547             
- Misses       306     308      +2     

Impacted Files	Coverage Δ
pyam/units.py	96.1% <ø> (-0.1%)	:arrow_down:
pyam/core.py	94.5% <33.3%> (-0.3%)	:arrow_down:
pyam/compute.py	100.0% <100.0%> (ø)
tests/test_feature_debiasing.py	100.0% <100.0%> (ø)

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