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

[sususu5]

Fixes #

Proposed Changes

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

Any background context you want to provide?

URL of the removed line in unit test https://github.com/STEllAR-GROUP/hpx/blob/1dcd2571f809830809a7f9d4ecdce7f2b555ca2e/libs/core/algorithms/tests/unit/algorithms/nth_element_sender.cpp#L90

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.

[StellarBot]

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[codacy-production[bot]]

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 (330b2eadf7294103c8610c805d7f4d3dbe301160)	193840	165029	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 (#6720)	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.