Remove map_regions() and default mappings csv

[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 removes the (not in use any more) method map_regions() method and a default-mappings csv file. This functionality is now taken over by the nomenclature.RegionProcessor class. A reference to the nomenclature user guide is added to the aggregate_region() documentation.

Closes #765

[codecov]

Codecov Report

Merging #773 (074c51e) into main (525ede9) will decrease coverage by 0.2%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##            main    #773     +/-   ##
=======================================
- Coverage   94.6%   94.5%   -0.2%     
=======================================
  Files         62      62             
  Lines       6107    6027     -80     
=======================================
- Hits        5779    5696     -83     
- Misses       328     331      +3     

Files Changed	Coverage Δ
pyam/core.py	95.0% <0.0%> (-0.2%)	⬇️
pyam/run_control.py	72.0% <ø> (ø)
tests/test_core.py	100.0% <ø> (ø)

... and 2 files with indirect coverage changes

[gidden]

Thanks @danielhuppmann - lgtm