Refactor initialization for simpler maintenance

[coroa]

Please confirm that this PR has done the following:

• [ ] Tests Added
• [ ] ~Documentation Added~
• [ ] ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

~On-top of #729.~ Rebased to main.

Splits utils.format_data into 6 different functions:

1. _convert_r_columns(df) - Check and convert R-style year columns
2. _knead_data(df, **kwargs) - Replace, rename and concat according to user arguments
3. _format_from_database(df) - Post-process database results
4. _intuit_column_groups(df, index) - Check and categorise columns in dataframe
5. _format_data_to_series(df, index) - Convert a long or wide pandas dataframe to a series with the required columns
6. _validate_complete_index(df)

Functionally it should be neutral. Make sure to look at individual commits to follow the refactoring trail.

[codecov[bot]]

Codecov Report

Merging #730 (58eb241) into main (7a97516) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #730   +/-   ##
=====================================
  Coverage   95.0%   95.0%           
=====================================
  Files         59      59           
  Lines       6014    6020    +6     
=====================================
+ Hits        5717    5725    +8     
+ Misses       297     295    -2     

Impacted Files	Coverage Δ
pyam/core.py	95.4% <100.0%> (ø)
pyam/utils.py	92.6% <100.0%> (+0.7%)	:arrow_up:

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[gidden]

This LGTM but best if @danielhuppmann approves!

[gidden]

LGTM - thanks @coroa ! Will merge after tests pass.