update phusion base image to 18.04

See merge request !2220

Feature/tests convergence 1p aniso

Closes #907

See merge request !2223

[md][facet][box] allow for mixed bcs

See merge request !2225

Feature/move geometry

Closes #903

See merge request !2219

[doxygen][mineralization] add missing opening parentheses in solid storage term in all min-models

See merge request !2214

[common][partial] Add MTBV containing const references

See merge request !2215

[math] Add linear table interpolation with two arguments

See merge request !2216

[readme] Add website's getting started guide to doc ressources

See merge request !2213

Fix macro calls and changelog entry

See merge request !2211

Replace calls to deprecated `DUNE_VERSION_NEWER_REV(DUNE_ISTL,2,7,1)`
by `DUNE_VERSION_GT_REV(DUNE_ISTL,2,7,0)` which also reads better than
the `GTE` version, as Dune 2.7.1 hasn't been released yet. Fix the
call to `DUNE_VERSION_GT(DUNE_ISTL,2,7)` in the solver factory
backend which would break things in case of a 2.7.1 release. Adapt
the corresponding changelog entry.

[changelog] Add comment on ISTL solverfactory

See merge request !2209

Feature/improve extract line data

See merge request !2207

* Make make dir of output dir more consise and handle "  " case
* simplify verbose output

Feature/staggered assembler fix forwarding implicit flag

See merge request !2206