The latter was reported N. Grunwald and should be fixed with the added
fPIC option in the VTK Conan package.

This makes sure that CMAKE_BUILD_TYPE variable is set which is
required by Conan.

Set OGS_USE_CONAN=ON to enable this. Once enabled there is no
conan install .. call necessary anymore.

You can select individual libraries to be used from the system, e.g.:

cmake -DOGS_USE_CONAN=ON -DOGS_LIB_VTK=System ..

This lets Conan install all third-party libraries except of VTK.

Fixes Eigen finding.

Local installation of libraries can be done with Conan.

On making tests fails with:

../bin/testrunner: error while loading shared libraries: libomp.so:
    cannot open shared object file: No such file or directory

Add error message when trying to build Gui with CMake 3.9.x.

Various fixes from #1907

[web] Fix binary files not commited with git-lfs

Related:
- #1887
- https://stackoverflow.com/a/340683/80480

This slipped through as currently macOS is not tested via CI.

This reduces download times significantly as downloaded decreases
from over 1 GB to around 60 MB.

Closes #1906

in future CMake version. Fixes a warning on CMake 3.9.

Use png for images. Fix sections in the markdown.

Vector extrapolation part2