WIP: Linear multistep Patankar schemes

[SKopecz]

This implements #107.

• Out-of-place implementation of the 2nd order MPLM22 scheme.
• In-place implementation of the 2nd order MPLM22 scheme.
• All other implementations
• Tests
• Docs
• Additional test problems
• Benchmark diffusion test problem

[SKopecz]

All the onestep MPRK/SSPMPRK and also the multistep Patankar schemes can be interpreted as sequences of MPE steps with slightly modified input data. Before continuing this PR we should implement a function like _permform_step_MPE! and rewrite the implementations of MPRK and SSPMPRK schemes. This will make the implementation of the multistep Patankar schemes easier and cleaner. The corresponding PR is #183 .

[codecov]

Codecov Report

❌ Patch coverage is 41.37931% with 85 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mplm.jl	0.00%	85 Missing ⚠️

📢 Thoughts on this report? Let us know!

[coveralls]

Pull Request Test Coverage Report for Build 21640780515

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 60 of 145 (41.38%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-4.1%) to 93.384%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/mplm.jl	0	85	0.0%

Totals
Change from base Build 21465719007:	-4.1%
Covered Lines:	1849
Relevant Lines:	1980

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💛 - Coveralls

[SKopecz]

The new problems need to be added here, to be found in the tests.