Allow passing filters when reading from an ixmp4 platform

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• ~Name of contributors Added to AUTHORS.rst~
• [x] 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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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 ```

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[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.