Compiler and linker errors on Euler

The compilation unit vtkwriter.cc uses VTK headers, but the target does not link VTK. This leads to error messages when trying to compile on Euler.

In addition, on Euler:

• The test executables need to link gtest.
• The executables that use std::filesystem need to link stdc++fs.
• The target/library pugixml-static cannot be found.

Open tasks:

• Confirm that changes do not break the Ubuntu Focal build (CI).
• Code review.