Adjust .wb.assign_income_groups() for sdmx 2.14.0

[khaeru]

sdmx1 v2.14.0 was released 2024-02-20 with improved but stricter handling of query parameters.

This prompted failures like this in the "pytest" CI workflow, because .wb.assign_income_groups() used a parameter name "startperiod" that was neither snake_case ("start_period") nor lowerCamelCase ("startPeriod").

The PR corrects.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• [x] Add or expand tests; coverage checks both ✅
• ~Add, expand, or update documentation.~ N/A, bugfix only
• ~Update doc/whatsnew.~

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d73fb4e) 75.4% compared to head (5fda86c) 75.4%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #154 +/- ## ===================================== Coverage 75.4% 75.4% ===================================== Files 93 93 Lines 6211 6211 ===================================== Hits 4684 4684 Misses 1527 1527 ``` | [Files](https://app.codecov.io/gh/iiasa/message-ix-models/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa) | Coverage Δ | | |---|---|---| | [message\_ix\_models/tools/wb.py](https://app.codecov.io/gh/iiasa/message-ix-models/pull/154?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa#diff-bWVzc2FnZV9peF9tb2RlbHMvdG9vbHMvd2IucHk=) | `100.0% <ø> (ø)` | |