Add option to set defaults in binary operations

[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 keyword argument to set default values in the binary operations if either a or b (or any keyword argument for apply()) are not defined. This can be used either as fillna=5 or fillna=dict(a=5}.

[codecov]

Codecov Report

Merging #538 (7ea19cc) into main (87ed604) will increase coverage by 0.0%.
The diff coverage is 97.2%.

@@          Coverage Diff          @@
##            main    #538   +/-   ##
=====================================
  Coverage   93.5%   93.5%           
=====================================
  Files         47      47           
  Lines       5129    5167   +38     
=====================================
+ Hits        4796    4832   +36     
- Misses       333     335    +2     

Impacted Files	Coverage Δ
pyam/_ops.py	96.0% <93.7%> (-4.0%)	⬇️
pyam/core.py	92.6% <100.0%> (ø)
tests/test_ops.py	100.0% <100.0%> (ø)

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[danielhuppmann]

Merging as announced via the Slack channel (unfortunately, no volunteers for a review)