Add implementation for to_netcdf()

[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 refactors #898 to use the internal pd.Series object for efficiency.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.0%. Comparing base (ddbb88e) to head (266ef92).
Report is 42 commits behind head on main.

Files with missing lines	Patch %	Lines
pyam/netcdf.py	83.3%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #899    +/-   ##
======================================
  Coverage   95.0%   95.0%            
======================================
  Files         64      66     +2     
  Lines       6134    6387   +253     
======================================
+ Hits        5828    6074   +246     
- Misses       306     313     +7     

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[danielhuppmann]

Merging to make a new release before the end of the year and send announcements before the break