Show all missing rows for require_data()

[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 changes the returned dataframe of the require_data() method to not only show the failing scenarios (dataframe of model-scenario combinations), but a dataframe that has all missing combinations of elements.

So if model_a - scen_a does not have the variableEmissions|CO2 at the global level, the method will return the following:

model	scenario	region	variable
model_a	scen_a	World	Emissions|CO2

closes #768

[codecov]

Codecov Report

Merging #772 (6755989) into main (9c947a8) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #772     +/-   ##
=======================================
- Coverage   94.4%   94.4%   -0.1%     
=======================================
  Files         62      62             
  Lines       6130    6132      +2     
=======================================
  Hits        5789    5789             
- Misses       341     343      +2     

Files Changed	Coverage Δ
pyam/core.py	95.1% <100.0%> (-0.1%)	⬇️
tests/test_feature_validation.py	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[phackstock]

One small comment other than that looks good to me.
I've tried to wrap my head around require_data in its entirety but wasn't quite able to. As long as it's working though that's fine.

[phackstock]

Very nice, that should help a lot in finding missing data 👍.
Good to merge from my side.