Remove C code from the dft_tools project

[harrisonlabollita]

Summary

This PR ports the C++ code in dft_tools to pure Python. Specifically, it replaces the linear tetrahedron integration routine from the VASP converter—used to calculate the density of states—with a Python implementation. Since this routine is not performance-critical, a Python version improves maintainability without impacting efficiency.

As a result, the dft_tools repository is now a Python + Fortran (via f2py) project.

Key Changes

• Removed legacy C/C++ code from c++/triqs_dft_tools/converters/vasp/.
• Reimplemented the linear tetrahedron integration routines in Python at python/triqs_dft_tools/converters/vasp/plovasp/lintetra.py
• Updated the corresponding linear tetrahedron tests.

[the-hampel]

Hi, this looks good to me. I added a numpy vectorized form of the tetrahedron. In my tests this is only a little slower than the C implementation. For 9 bands (5d orbitals NiO) and a 11x11x11 mesh of kpoints (1331) on 3001 grid points I get on my m3 macbook:

33x/log: Elapsed time: 15.8411 seconds       
rewrite_py/log: Elapsed time: 24.3028 seconds

for the integration call. Given that this is only used to verify a setup, and should also greatly improve when symmetries are used I think we can safely replace the C code to make dft_tools python only. We can also leverage trivially mpi parallelism if we wish in the calling function in the future.

@opeil any objections?

We should maybe enforce Python support then in this build @Wentzell does this make sense? Currently there is still some code like this:

if(PythonSupport)
 add_subdirectory(python)
 add_subdirectory(python/plovasp)
 add_subdirectory(python/elk)
endif()

in the cmake files.

Best,
Alex

[the-hampel]

I removed now all C++ dependencies in cmake and also rebased this branch on top of dft_tools/unstable (was previously 3.3.x) . Can you have a look @Wentzell if I removed the right things (I removed also dependency on cpp2py). Thanks!

[Wentzell]

I pushed some additional cleanups for the project structure / CMake. This looks good to be merged.