Google test for unit tests

Closes #152

See merge request !159

* Include latest version of Google Test as Git Submodule.
* Link Google Test to all DORiE unit tests. This has no effect if its
  header file is not included.
* Enable testing in DORiE if dune-testtools is found.
* Update SimulationBase unit test to work with Google Test.
* Add Doxygen docs on unit testing.
* Update README.md.

Merge branch '154-linear-interpolator-only-works-for-minimal-shapes-and-trivial-extensions' into 'master'

Resolve "Linear interpolator only works for minimal shapes and trivial extensions"

Closes #154

See merge request !156

Fetch the enclosing multi indices with the coordinates in mesh units,
not physical units. Improve the unit test to cover more complicated
cases, thus ensuring that the interpolator works as intended.

Resolve "Add a Finite Volume local operator for the Richards equation"

Closes #137

See merge request !132

Const all the things!

* Mention FV and DG solver options in Feature Overview.
* Update default parameter descriptions on 'FEorder' and 'vertex data'.
* Adjust Richards FV local operator docstrings
  - Add RichardsDGTemporalOperator to Richards model doc module.
  - Add [out] tags to residual arguments.
  - Use regular comment scheme when comments surpassed line limit.

[Doc] Fix Gauss equation in initial condition documentation

See merge request !154

Also sanitize the variable for the variance and fix the quantity of
the initial condition which depends on the spatial dimensions.

Resolve "Add linear interpolator and use it for scaling fields"

Closes #121

See merge request !145

Update rfg to randomfield v2.6

Closes #150

See merge request !153

Meta-ini system tests runs all converters and two covariance options
in 2D and 3D and writes all into the same output file.

* Add meta-ini file for test and register it.
* Add empty dorie_pfg testtools Python module.
* Build dorie-rfg executable in CI system-tests build job.

* Adapt new template signature of RandomField class.
* Convert indentation to spaces.
* Remove unnecessary includes.
* Switch from 'switch' to 'if...else'.

Resolve "DORiE installation not working with newest dune-randomfield master"

Closes #150

See merge request !151

* Update Dockerfile for base image
* Bump Docker base image to v1.2
* Update version info in README.md
* Update CHANGELOG.md

* Create docs page for interpolators.
* Add more information on difference between nearest-neighbor and
  linear interpolator.
* Fix explanation of required interpolator keys in parameter file docs.

Revert "Merge branch 'hotfix-sphinx-version' into 'master'"

See merge request !152

This reverts merge request !148

Use VTKReader as dictionary

See merge request !150

Resolve "Add parameterization for the effective hydrodynamic dispersion tensor"

Closes #142

See merge request !141

Merge branch '142-add-parameterization-for-the-effective-hydrodynamic-dispersion-tensor' of ssh://ts-gitlab.iup.uni-heidelberg.de:10022/dorie/dorie into 142-add-parameterization-for-the-effective-hydrodynamic-dispersion-tensor

Increase contents tree depth.

[skip ci]

[skip ci]

* This allows sphinix to create an index which is easier to navegate