Extend Excel write/read to handle large data beyond the row limits

[behnam-zakeri]

In response to issue #292, this PR extends the code in ixmp.backend.io.py to write and read large data, i.e., dataframes and series longer than the maximum row number of Excel.

How to review

You can test this branch as it is. The tests are improved to reflect the new changes. ALternatively, you can export one of the global scenarios and check if parameter land_output is completely transferred to Excel in two sheets.

PR checklist

• Tests improved to reflect the enhancements.
• Documentation is not needed, minor enhancement.
• Release notes updated by adding a short note.

[codecov]

Codecov Report

Merging #309 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
+ Coverage   97.31%   97.41%   +0.10%     
==========================================
  Files          39       41       +2     
  Lines        4090     4251     +161     
==========================================
+ Hits         3980     4141     +161     
  Misses        110      110              

Impacted Files	Coverage Δ
ixmp/backend/base.py	98.65% <100.00%> (ø)
ixmp/backend/io.py	98.55% <100.00%> (+0.11%)	⬆️
ixmp/cli.py	98.67% <100.00%> (+<0.01%)	⬆️
ixmp/core.py	95.80% <100.00%> (-0.01%)	⬇️
ixmp/tests/test_cli.py	100.00% <100.00%> (ø)
ixmp/reporting/key.py	100.00% <0.00%> (ø)
ixmp/reporting/quantity.py	95.00% <0.00%> (ø)
ixmp/tests/reporting/__init__.py	100.00% <0.00%> (ø)
... and 6 more

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[zikolach]

I left some comments

[zikolach]

Wouldn't it better to always add numeric prefix if number of elements is more than 1 sheet?

[behnam-zakeri]

Now, the sheets will be saved as foo, foo(1), foo(2), ... . I think it's better to see the sheets starting with the name of the items (in case the sheet name is not fully expanded).

[khaeru]

In addition to the points raised by @zikolach, the documentation (current version shown here: https://message.iiasa.ac.at/projects/ixmp/en/master/file-io.html#scenario-model-data) needs to be adjusted to describe the new format.

[behnam-zakeri]

In addition to the points ... needs to be adjusted to describe the new format.

Thanks @khaeru, I extended the doc to reflect this. I also noticed a redundant sentence and deleted that.

[khaeru]

Excellent @behnam-zakeri, thank you!

I used the Python built-in functions range(), zip(), and enumerate(), plus the technique of defining a function-within-a-function, to streamline the code a little, but your improvements were already very tidy, and the addition of tests is a solid example.

Will merge once tests pass.

[behnam-zakeri]

Thanks @zikolach for useful comments and @khaeru for very nice improvements of the code.