Draft: Feature/new experimental assembly

What this MR does / why does DuMux need it:

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Notes for the reviewer

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Before you request a review from someone, make sure to revise the following points:

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.
• (if not applicable remove) are newly introduced or modified physical values/functions backed up with a scientific reference (including doi) in the docs?
• (if not applicable remove) if the examples are modified, is the documentation regenerated (using generate_example_docs.py)

potential steps/milestones:

• use some external lib for solution of the linear system (e.g. Eigen) -> makes sure the layout is flexible enough to make use of non-dune-solvers
• set up a toy example with time dependency to properly integrate TimeStepper into the new assembly layout.
• set up a toy multidomain example to ensure that support for multitypeblockvectors/matrices can be realized
• implement actual assembler and realize poisson problem
• make stationary 1p test work
• make instationary test work
• make a 2p2c test work (privarswitch should be tried to be put into GridVariables::update() such that newton is agnostic of that
• check that multidomain test with privar switch works with the new assembly layout