cleanup/staggeredupwindfluxvars ifconstexpr

See merge request !2127

[freeflow][staggeredupwindfluxvars] reduce the doLateralMomentumUpwinding_ functions with if constexpr

[freeflow][staggeredupwindfluxvars] reduce the getLateralUpwindingMomental_ functions with if constexpr

[freeflow][staggeredupwindfluxvars] reduce the getFrontalUpwindingMomenta_ functions with if constexpr

[freeflow][staggeredupwindfluxvars] reduce the doFrontalMomentumUpwinding_ functions with if constexpr

[cleanup] Remove unused typedefs

See merge request !2124

Use Markdown for auto-generated README.md

See merge request !2121

Cleanup rans tests with c++17

See merge request !2119

[cleanup][2p] Remove include of 1p model

See merge request !2122

[docu][doxygen] Change dot image format from png (default) to svg to reduce...

Closes #835

See merge request !2123

[docu][doxygen] Change dot image format from png (default) to svg to reduce the size of the doxygen html folder to less than 40 percent.

Although `extractmodulepart.sh` generates a `README.md`, no
Markdown is used so far in this file. Use Markdown to define
headings, lists and a table.

[facet][box] do not use modified elembctypes

See merge request !2115

[fix] Update cmake install headers

See merge request !2117

[changelog] Comment on support for pre 2.12

addresses comment in #827

See merge request !2116

This commit also modifies the tracer_tracer test, as this failed for box
after removing the elembctypes. This is because it was using Neumann
no-flow boundary conditions for the fracture domain on the left boundary
while the bulk problem used Dirichlet BCs. The old elembctypes for box
were discarding Dirichlet BCs on those nodes on the boundary that also
couple to fractures in order for the coupling conditions to be
fulfilled. However, that discards the boundary condition. With this
commit, Dirichlet BCs are always fulfilled, but for the tracer test this
means that as a consequence, the pressure in the fracture on the left
side is equal to the bulk pressure - because it uses Dirichlet-type
coupling conditions. This commit changes the test such that it uses
Dirichlet BCs in the bulk domain only on the lower part of the boundary
such that the fracture touches the domain boundary at a no-flow
boundary.

Fix warning after DUMUX_REGISTER_PRECONDITIONER superfluous semicolon

Occurs only with pedantic

Needs backport to 3.2

See merge request !2110

[tests][richards] Replace HAVE_UG in CMakeLists

See merge request !2109

[doc][fixup] Update handbook 6_parallel.tex

See merge request !2108

[cleanup][handbook] update parallel section with new linear solver

See merge request !2106