Although `extractmodulepart.sh` generates a `README.md`, no
Markdown is used so far in this file. Use Markdown to define
headings, lists and a table.

[facet][box] do not use modified elembctypes

See merge request !2115

[fix] Update cmake install headers

See merge request !2117

[changelog] Comment on support for pre 2.12

addresses comment in #827

See merge request !2116

This commit also modifies the tracer_tracer test, as this failed for box
after removing the elembctypes. This is because it was using Neumann
no-flow boundary conditions for the fracture domain on the left boundary
while the bulk problem used Dirichlet BCs. The old elembctypes for box
were discarding Dirichlet BCs on those nodes on the boundary that also
couple to fractures in order for the coupling conditions to be
fulfilled. However, that discards the boundary condition. With this
commit, Dirichlet BCs are always fulfilled, but for the tracer test this
means that as a consequence, the pressure in the fracture on the left
side is equal to the bulk pressure - because it uses Dirichlet-type
coupling conditions. This commit changes the test such that it uses
Dirichlet BCs in the bulk domain only on the lower part of the boundary
such that the fracture touches the domain boundary at a no-flow
boundary.

Fix warning after DUMUX_REGISTER_PRECONDITIONER superfluous semicolon

Occurs only with pedantic

Needs backport to 3.2

See merge request !2110

[tests][richards] Replace HAVE_UG in CMakeLists

See merge request !2109

[doc][fixup] Update handbook 6_parallel.tex

See merge request !2108

[cleanup][handbook] update parallel section with new linear solver

See merge request !2106

[examples] fix threshold for 2p infiltration example

See merge request !2104

[cleanup] Remove OpenMP dependency after upstream fix in opm-common/release/2020.04

See merge request !2102

[test][facet][1pnc] do not require UMFPack

See merge request !2100

[test][sequential] remove failing grid/solver combinations

See merge request !2099

[test][ff][ms] Use constant time step size

See merge request !2097

Fix/test saltwaterintrusion increase robustness

See merge request !2092

Cleanup/unused params

See merge request !2088

[kepsilon] Fix compiler warning complaining about const having no effect

See merge request !2089

[readme][changelog][license] correct typesetting of DuMux

See merge request !2086

[fix] Add virtual destructor to vtkoutputmodule base class

See merge request !2082