Feature/deprecate old preconditioner params

Closes #825

See merge request !1890

We do not need to support the old style parameters as this class was
just recently added on master.

As already noted in 76563518 the global
defaults have been replaced by local defaults. The change already
appears in the changelog. In case someone relied on the global defaults
in their own code they will get a parameter error at runtime which is
easy to fix.

[test] Explicitly tell gmsh to mesh in msh2 format

See merge request !1889

Feature/examples automate readme

Closes #824

See merge request !1888

[fix] Shadow template parameter error

See merge request !1887

[dune] Fix fixedSize interface using version check

See merge request !1886

Due to a bug in the upstream deprecation mechanism for "fixedsize"
(lowercase s), the transition cannot be done smoothly so we
use version checks.

Cleanup/deprecate optional

See merge request !1881

[cleanup] fixedsize -> fixedSize in data handle (upstream change)

See merge request !1883

Fix/vtkoutput constructor

Closes #571

See merge request !1838

Simon Emmert authored Feb 11, 2020 and Timo Koch committed Mar 15, 2020

Introduces the possibility to have variable precision, e.g. Float64.
Updates the arguments of the VTKOutputModule constructor and adds the
version check for dune. Default stays Float32.

Cleanup/remove deprecated

See merge request !1879