Fix bug in io.matpower.system2mpc with multiple PQ at one bus

[jinningwang]

No description provided.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨

[trag-bot]

Pull request summary

• Refactor system2mpc function to handle multiple PQ loads on the same bus by summing their contributions instead of overwriting them.
• Update the handling of shunt elements in system2mpc to similarly sum contributions from multiple shunts on the same bus.
• Modify the release notes to document the bug fix for handling multiple PQ loads in io.matpower.system2mpc.
• Add a new test case test_system2mpc_pq to verify the correct conversion of systems with multiple PQ loads to MPC format.
• Correct the formatting of function names in the release notes for consistency.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.04% (target: -10.00%)	✅ 100.00% (target: 0.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (718b67c)	9549	7309	76.54%
Head commit (9104905)	9565 (+16)	7325 (+16)	76.58% (+0.04%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#178)	21	21	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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