[embedded] Make sure integer does not wrap around for small ratios

Closes #942

See merge request !2293

Fix compiler warning related to implicit conversion

This makes sure, given some ratio that the resulting number of samples
is between 1 and the largest representable std::size_t.

The given ratio is clamped to the interval (0,1] if it's outside that range.

Feature/extensible tags

See merge request !2287

Fix output for 2p tracer test

See merge request !2290

The enums provide a convenient way to select the type of 1d3d coupling mode.
However they are not extensible. Adding a new mode requires changing Dumux core code.
With the new comparable and named tags, we have the same features but in
an extensible way. Every coupling manager implementation can define their own
(unique) tag and specialize the common class with this tag. The coupling mode
can be queried as before, and it is still easy to write code that works
with several coupling modes.

This commit also puts all coupling mode specializations in their own headers.

The VTK output of the 2p model has been called before
`timeLoop->advanceTimeStep()`, resulting in a wrong timestep
number in the pvd file. Fix by moving the VTK output call to the
correct position.

[test][channel] put Scalar alias outside subgrid guard

See merge request !2286

[doc][doxygen] Improve description of doxygen group geometry.

See merge request !2284

Feature/fix walldistance

See merge request !2224

Ned Coltman authored Oct 14, 2020 and Melanie Lipp committed Oct 16, 2020

[rans] Introduce a struct for the wallElementinformation, change HasChannelGeometry to IsFlatWallBounded, fix maximum/minimum velocity names (remove profile, add comments), flowNormalAxis to flowDirectionAxis,  sandgrainroughness default.

Ned Coltman authored Oct 09, 2020 and Melanie Lipp committed Oct 16, 2020

* Include non-  normal axis distances
* Cleanup wall and flow normal axis variables
* Include scvf corners as possible wall locations

Doxygen for 3.3

Closes #920

See merge request !2258

[test][bBoxTree] Adapt to change in Dune::AxisAlignedCubeGeometry

See merge request !2283

* for dim != dimworld, an axes object needs to be passed to the ctor

Revert "Merge branch Dune::power for std:pow and integer exponent"

See merge request !2279