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* Adjust toctree to have proper sections
* Add new doc pages:
  - Feature overview
  - Data I/O
* Add new doc section 'Introduction'
* Rename main file to `index.rst`

* Do not use DUNE CMake module for creating Sphinx docs anymore
* Create subdirectories: manual, python, cookbook
* Remove unused files
* Remove 'Guided doxygen documentation'
* Add symlink from doc/manual/ to README

Resolve "Add input of boundary segmentation"

Closes #123

See merge request !119

[ci skip]

Rename '[get_]element_index_map' and '[get_]boundary_index_map'

Resolve "Remove boundary conditions "evaporation" and "limitedInflux""

Closes #124

See merge request !120

[skip ci]

Remove evaporation and limited influx from docs

* remove options from input scheme
* remove options in Richards LOP
* remove grid view from LOP constructor arguments
* remove BC type cache from LOP

* Create new docs page for grid creation and mapping
* Move mapping docs to new page and adjust
* Add paragraphs for boundary mappings

Set volume to homogeneous index 0 and set boundaries to increasing
indices

* Update 'test-grid-creation': Check for correct boundary segmentation
* Add boundary segmentation datasets
* Add boundary segments to GMSH mesh files
* Update .mini files with new keys

* rename `grid.mappingFile` to `grid.mapping.file`.
* rename `grid.mappingFileDataset` to `grid.mapping.volume`.
* remove `grid.globalIndex`. Same functionality is provided by setting
    `grid.mapping.volume` to an integer index.
* add keys for boundary mapping datasets.

* Add nested class GridMapper::Boundary for helper functions regarding
    boundary side identification and handling
* Rewrite indexing functions of GridMapper<Grid, rectangular> to also
    work with lower-dim arrays

* Add function 'is_none' for checking if user input is 'none'
* Move functions 'to_lower' and 'get_grid_extensions' to new header

* Add module for grid creation
* Add author and date information
* Improve docstrings of classes and functions

Also, fix header in transport.hh

* GridMapper is now called internally by the constructor of GridCreator.
    The object is not returned by the creator anymore.
* Specialize GridMappers for 'gmsh' and 'rectangular' grids and
    derive them from GridMapperBase
* GridMapper now reads the mapping H5 file
* The grid is now retrieved from GridCreator
* Local element and boundary index maps are now retrieved from
    GridCreator
* Adapt simulations to expect GridCreator as argument

Minor changes:
* Store file name in H5File
* Add free function for lowercasing strings
* Add logger to GridMapper

Resolve "Add a logging framework for command line output"

Closes #92

See merge request !106

[ci skip]

* default logger for scaling is Richards
* pass logger to scaling adapters
* default logger for interpolators is base
* only InterpolatorFactory uses logger

Do not display the exception as 'error' message again

Rename groups.dox files to modules.dox

See merge request !118