Local file cache

[Rlamboll]

Done to resolve #681

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

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Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

Please describe the changes introduced by this PR.

[codecov]

Codecov Report

Merging #696 (4acc5dd) into main (4bd16a9) will increase coverage by 0.0%.
The diff coverage is 97.8%.

@@          Coverage Diff          @@
##            main    #696   +/-   ##
=====================================
  Coverage   94.8%   94.9%           
=====================================
  Files         58      58           
  Lines       5856    5900   +44     
=====================================
+ Hits        5557    5600   +43     
- Misses       299     300    +1     

Impacted Files	Coverage Δ
pyam/iiasa.py	86.8% <95.6%> (+0.7%)	⬆️
pyam/__init__.py	67.5% <100.0%> (ø)
tests/test_iiasa.py	97.8% <100.0%> (+0.2%)	⬆️

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[danielhuppmann]

Thanks for the PR @Rlamboll!

I'm worried about two issues:

1. By accepting only csv files for the local copy, you loose all meta indicators, which can be very relevant
2. The query is not actually stored, so if you do (as you do in the test

   df = lazy_read_iiasa("some/file.csv", TEST_API, model="model_a")

   and then

   df = lazy_read_iiasa("some/file.csv", TEST_API, model="model_b")

   you will get a different result for the second call depending on whether any data changed in the database in the meantime.

[Rlamboll]

I've enabled it to read xls(x) files, but I don't think it's possible to do proper checks that the filters are applied in only a more restrictive sense without spending a significant period of time querying the database, which is what we're trying to avoid in the first place. I've expanded the documentation to make it clear that it's on the user to look out for this. This is consistent with how the code is currently used.

[danielhuppmann]

I'm not 100% convinced about this new feature because there might be confusion when a user queries for different filters than what is stored in the local cache. I'll approve, merge, and then create an issue for future improvements.