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.

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

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

* Use regular math for scalars, bold for vectors, sans-serif for
  tensors.
* Replace superscripts by subscripts.
* Replace `\mathrm` by `\text`.
* Fix typos.

[skip ci]

System tests previously only excluded the unit tests, which lead to the
Python tests being executed for 'make system_tests'.

Add __init__ to pytest directory to enable package-local includes.

This enables convenient iteration over all data arrays in a VTK file,
even without their names given. These can now be retrieved from the
reader directly.

* Make VTKReader a mapping: Use it like dict to access data arrays.
* Wrap data arrays into VTKDataArray for evaluation.

* Remove `Type`to structs of parameters and add `Param` to those parameterizations which clash with parameters structs