That makes it possible to overload the local assembler class while
reusing an interaction volume.

instead of storing pointers to the transmissibilities and temperatures
etc, we now only store a pointer to the data handle and reconstruct
the fluxes in the FouriersLaw implementation. This dramatically reduces
the number of lines needed for the cache implementation.

instead of storing pointers to the transmissibilities and pressures
etc, we now only store a pointer to the data handle and reconstruct
the fluxes in the FicksLaw implementation. This dramatically reduces
the number of lines needed for the cache implementation.

instead of storing pointers to the transmissibilities and pressures
etc, we now only store a pointer to the data handle and reconstruct
the fluxes in the DarcysLaw implementation. This dramatically reduces
the number of lines needed for the cache implementation.

This should be identical for all mpfa schemes

That was used by the mpfa framework only, which also doesn't anymore.

All matrices are now stored in the data handles

Instead of deriving from the process-related handle classes the
handle class contains these sub-handles and provides access to it.
That allows process-independent algorithms on the sub-handle classes
as soon as the return functions get more generic names.

Resizing of the matrices place in the local assembler directly wherr
needed. This has a small negative impact on performance but saves a lot of code.

Not providing an overload for a function in the base class within
the interaction volume implementation would have led to an infinite
loop. We now throw if any of the functions are not overloaded.

[gridvars] Unify for stationary and instationary problems

See merge request !1306

[test][1p][quad] add missing namespace specifier

See merge request !1307

[ni] Remove unused alias

See merge request !1305

[test][navierstokesnc] Use different executables for caching on/off

See merge request !1303

Provide a script that replaces property macro usages

Closes #593

See merge request !1300

Feature/user specified solid params in spatial params

Closes #578

See merge request !1231

[propertysystem] include propertysystemmacros.hh again

See merge request !1299