[porousmediumflow][velocity] Improve calculation in case of Neumann BCs

See merge request !2129

* check if we have non-compositional model
* estimate the volume flow by neumannFlow/density

[porousmediumFlow] Export indices in all volVars

See merge request !2229

[docu][freeflow] Improve rans oneeq docu.

See merge request !2251

[material][2p] Move thermalconductivity laws to their own folder

See merge request !2249

[md][embedded] Fix CMakeLists.txt

See merge request !2247

[common] Add inverse of monotone cubic spline

See merge request !2250

* Add missing header

[basegridgeometry] Use more robust way to choose ctor of mappers

Closes #932

See merge request !2245

* use std::is_constructible

[readme] Update link to class documentation

See merge request !2242

[common] make sure dt != 0 in timeloop

Closes #928

See merge request !2237

- don't insert checkpoints at current time
- warn when dt set to 0 and not finished

[cleanup][test] Rename test_md_embedded{dims} to test_md_embedded_{dims}

See merge request !2241

Fix/cc internal dirichlet

Closes #929

See merge request !2240

[bugfix] Fix compiler error introduced in !2236

See merge request !2238

Fix/navier stokes rot sym

Closes #927

See merge request !2236

[common] fix segment index issue in spline extrapolation

Closes #925

See merge request !2235

Resolve "doc shallow water flux"

Closes #924

See merge request !2234