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

hpx/libs/core/algorithms/tests/unit/algorithms/nth_element_sender.cpp

Line 90 in 1dcd257

nth_element_sender_test<std::forward_iterator_tag>();

Checklist

Not all points below apply to all pull requests.

• 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.
• 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]

Can one of the admins verify this patch?

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.70%	✅ 77.42%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (15afc8b)	244963	210174	85.80%
Head commit (330b2ea)	193840 (-51123)	164950 (-45224)	85.10% (-0.70%)

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)	31	24	77.42%

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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