Feature/navier stokes darcy convtest

See merge request !2128

[test] Use python3 for test wrappers

See merge request !2138

[cmake] Require CMake 3.13

See merge request !2136

* based on Schneider et al. 2019:
  Coupling staggered-grid and MPFA finite volume methods for
  free flow/porous-medium flow problems

Export element type  in FV element geometry

See merge request !2132

Fix/staggered upwindfluxvars

See merge request !2133

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.