Allow passing filters when reading from an ixmp4 platform

[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 allows passing filters when reading from an ixmp4 platform and it extends read_iiasa() to first try reading from ixmp4 platforms hosted by IIASA.

[codecov]

Codecov Report

Attention: Patch coverage is 85.18519% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 95.0%. Comparing base (ddbb88e) to head (7bfe5d8).
Report is 6 commits behind head on main.

Files	Patch %	Lines
pyam/iiasa.py	66.6%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #838     +/-   ##
=======================================
- Coverage   95.0%   95.0%   -0.1%     
=======================================
  Files         64      63      -1     
  Lines       6134    6150     +16     
=======================================
+ Hits        5828    5843     +15     
- Misses       306     307      +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[phackstock]

Looks good to me overall.
Some minor comments in line.

[danielhuppmann]

Responding here to a larger comment by @phackstock in the review: this PR only touches the pyam.read_iiasa() method, and if there are ixmp databases and ixmp4 platforms with the same name, the default is now using ixmp4.

But a user will still have the possibility to make that selection explicitly either using:

df_ixmp = pyam.iiasa.Connection(name).query(**filters)
df_ixmp4 = pyam.read_ixmp4(name, **filters)

It is our responsibility to keep potential overlaps of "database existence" to a minimum and keep the actual data consistent between new and legacy instances.

[phackstock]

@danielhuppmann sounds good to me.

[phackstock]

Good to be merged from my side