Minimum version: 3.2

[CMake] Remove external projects

Possible values: missing (default), all, never, list of lib names

Examples:

cmake . -DOGS_CONAN_BUILD="Eigen3;Boost" -> Builds Eigen and Boost locally
cmake . -DOGS_CONAN_BUILD=all            -> Builds all libs locally

This basically reimplements all functionality we had with ExternalProject.

My Boost fix was integrated in 1.64.0 only.

Added Conan remote conan-community where the package is hosted.

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.