Add unit tests for using binary ops with numerical args

[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

PR #533 implicitly generalized the binary-operation methods to enable using numbers as arguments. This PR adds unit tests to make sure this continues to work as expected, updates the docstrings, and adds a sanity check if a user would do something like df.add(2, 3, "foo"), where the return-object cannot be cast to an IamDataFrame.

closes #530

[danielhuppmann]

pinging @pjuergens for a review

[codecov[bot]]

Codecov Report

Merging #537 (7fd55c6) into main (3684c24) will increase coverage by 0.0%. The diff coverage is 92.3%.

@@          Coverage Diff          @@
##            main    #537   +/-   ##
=====================================
  Coverage   93.4%   93.5%           
=====================================
  Files         47      47           
  Lines       5123    5129    +6     
=====================================
+ Hits        4789    4796    +7     
+ Misses       334     333    -1     

Impacted Files	Coverage Δ
pyam/utils.py	92.2% <25.0%> (+0.2%)	:arrow_up:
pyam/_ops.py	100.0% <100.0%> (ø)
pyam/core.py	92.6% <100.0%> (ø)
tests/test_feature_compare.py	100.0% <100.0%> (ø)
tests/test_ops.py	100.0% <100.0%> (ø)

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[danielhuppmann]

Merging without review (because this PR is mainly adding tests)