Handle optional dependency MPI by directly passing HAVE_MPI flag

Summary

This MR changes the way optional dependencies are enabled in cmake, following the MR !1294 (merged). Essentially the add_dune_xy_flags function and the dune_register_package_flags now sets HAVE_XY directly instead of writing a config.h entry and setting ENABLE_XY afterwards.

In this MR the package flags for MPI are updated.

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• 95957ad7 - Handle optional dependency MPI by directly passing HAVE_MPI flag

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mentioned in issue #355

changed milestone to %CMake Modernization

Is this compatible with testing? When we moved from Autotools to CMake, the main reason to keep the ENABLE_<feature> trick was the we wanted to test some features and their fallback solution. I remember the case for SuperLU, where we wanted to test also the iterative fallback, like in PDELab.

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• 95957ad7...f7f70723 - 2 commits from branch master
• d7db78ae - Handle optional dependency MPI by directly passing HAVE_MPI flag

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• 2f79227d - Remove HAVE_MPI from dune_python_configure_bindings

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• 43b7527b - Debug CI with artifact

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• e81e8bc9 - Revert "Debug CI with artifact"

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• c4009a16 - Revert: Remove HAVE_MPI from dune_python_configure_bindings

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• 14bee5dd - Pass enable flag for MPI directly to compiler

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I have cleaned up the commit history (and changed the commit message)

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enabled an automatic merge when the pipeline for 14bee5dd succeeds

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merged

mentioned in commit f7336f47

mentioned in merge request !1314 (merged)

mentioned in merge request !1317 (merged)

mentioned in issue #357