[fix][doc] Fix comment

See merge request !987

Cleanup/staggered clang warnings

See merge request !986

[3p][fluidsystem][fix] Remove exceptions from constexpr functions. Make more functions constexpr.

Closes #494

See merge request !984

[cleanup] Remove extract dof privars in volumevariables. Extract directly from elemsol.

Closes #492

See merge request !985

[fv] State the global coordinate type in the grid geometry

See merge request !982

Feature/improve ff boundary

Closes #487

See merge request !953

* index of the pressure has to equal the one of the main component
* advection law does not need outflow information
* fix some reference solutions

* not a valid combination

* these gridVolVars are free-flow specific

Feature/poroelasticity

See merge request !981

Some of the interfaces were designed such that they are in agreement
with the porous medium flow framework where we make them work for
both cell-centered as well as the box scheme. In the geomechanics
framework we now make use of the fact that cell-centered schemes are not
considered and use more FEM-like interfaces for the parameters required
for stress evaluations. This might enable us to rename/redefine these
interfaces to work with both FEM as well as Box. However, if
cell-centered schemes for geomechanics are to be implemented one day,
the whole structure would have to be overthought.

[newton] Add report function for statitics output

See merge request !975

Cleanup/indentations

See merge request !977