Is enabled when OGS_BUILD_GUI=ON

We currently stick to Snakemake.

The usage of CPMFindPackage instead of CPMAddPackage searches for
local installs of the package first and falls back to CPM if not
found.

Submodule commit b0bc24cfdda39b30145b1f6c2c84b801cd131fef was not available anymore in the master branch of ArashPartow/exprtk.
The branch was maybe rebased? Used commit c7c219480d9678eec7383a4a99030683c4a84d91 instead (same commit date).

Test runs on envinf1.

Reverted custom naming for xdmf libs.
Added Iconv as a dependency (dependency of libxml2).

Because vtkIOXdmf3 is include the symbols are found there.
This commit fixes problem when vtkIOXdmf3 will be removed (soon)
We will remove vtkIOXdmf3 soon

This avoids dependency clashes with system-installed metis.

- add include paths for Xdmf and libxml2 from VTK
- disable xdmf test on Apple Clang

https://gitlab.kitware.com/vtk/vtk/-/issues/17353

 added static link for vtkIOXdmf3

Choose the tfel installation for usage in Conan by setting TFELHOME
CMake- or environment variable. Is automatically set when using
Conans tfel-package.

Fixes #3034.