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

[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.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 73 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mplm.jl	0.00%	73 Missing ⚠️

📢 Thoughts on this report? Let us know!

[coveralls]

Pull Request Test Coverage Report for Build 20463325235

Details

• 0 of 73 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-3.3%) to 94.514%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/mplm.jl	0	73	0.0%

Totals
Change from base Build 20199034964:	-3.3%
Covered Lines:	2050
Relevant Lines:	2169

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💛 - Coveralls