Refactor to use validation module

[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 does three things:

• move the utility-method make_index() to the utils module
• move the utility-method _exclude_on_fail() to a new validation module
• add tests to show that require_data() implements logical-and, i.e., all combinations across dimensions are required, (see Support logical-and in require_data() #768

[codecov]

Codecov Report

Merging #771 (2dd765d) into main (f8d0ca0) will increase coverage by 0.0%.
The diff coverage is 97.5%.

@@          Coverage Diff          @@
##            main    #771   +/-   ##
=====================================
  Coverage   94.4%   94.4%           
=====================================
  Files         61      62    +1     
  Lines       6121    6130    +9     
=====================================
+ Hits        5780    5789    +9     
  Misses       341     341           

Files Changed	Coverage Δ
tests/test_core.py	100.0% <ø> (ø)
pyam/core.py	95.2% <91.6%> (-0.1%)	⬇️
pyam/utils.py	90.7% <100.0%> (+0.2%)	⬆️
pyam/validation.py	100.0% <100.0%> (ø)
tests/test_feature_validation.py	100.0% <100.0%> (ø)

[phackstock]

Looks good to me. Two small comments below.

[phackstock]

I'd suggest renaming the function s into something more descriptive.

[danielhuppmann]

Any good ideas? The point of that method is to having something short that can be used in a docstring...

[phackstock]

Maybe plural_s? Although I typed that before I'd seen the use in a docstring. There it's quite readable, so ok from my side to keep it this way if we don't have any good ideas.

[phackstock]

Good to be merged from my side.
If you find a good alternative name for the s function you can change it otherwise it's fine as well to keep it as is and merge.