Fixed the S/R version of partial_sort with unit test added

[sususu5]

Fixes #

Proposed Changes

• Add scheduler-specific implementation for the parallel version of the partial_sort algorithm
• Unit test from last year's GSoC project is added and passes
• A line testing forward iterator on partial_sort is removed from the test case because it does not conform to the C++ standard

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Checklist

Not all points below apply to all pull requests.

• [x] I have added a new feature and have added tests to go along with it.
• [ ] I have fixed a bug and have added a regression test.
• [x] I have added a test using random numbers; I have made sure it uses a seed, and that random numbers generated are valid inputs for the tests.

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Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 15afc8be41b0668584dfafe2d8cf6ffb75dfb715[^1]	:white_check_mark: ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (15afc8be41b0668584dfafe2d8cf6ffb75dfb715)	Report Missing	Report Missing	Report Missing
Head commit (0c64d303e7d6aac2df59153b7b9587723860a9d9)	193839	165032	85.14%

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 (#6727)	0	0	∅ (not applicable)

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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[^1]: Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct.