Add a tutorial for working with algebraic operations

[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 adds a tutorial for algebraic operations, which is rendered here.

Also, while writing the tutorial, I noticed that the ignore_units feature didn't quite sufficiently ignore units, so I added a fix.

[danielhuppmann]

Thanks @jkikstra, fixed!

[codecov[bot]]

Codecov Report

Merging #543 (64db5a4) into main (8d8aa6b) will decrease coverage by 0.0%. The diff coverage is 92.8%.

@@           Coverage Diff           @@
##            main    #543     +/-   ##
=======================================
- Coverage   93.5%   93.5%   -0.1%     
=======================================
  Files         47      47             
  Lines       5228    5238     +10     
=======================================
+ Hits        4891    4900      +9     
- Misses       337     338      +1     

Impacted Files	Coverage Δ
pyam/_ops.py	94.0% <75.0%> (-1.0%)	:arrow_down:
tests/test_ops.py	100.0% <100.0%> (ø)

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