CEH gradients

[Albkat]

The implementation of the numerical charge gradients num_grad_chrg (for CEH).
The output is written to ceh.grad.
@lukaswittmann, please be careful with the output format.

[Albkat]

The CI failure is due to the missing LAPACK/BLAS libraries in Ubuntu 24.04.1.

It seems to work with Ubuntu 24.04.5 (run)
As the updated runners are still being rolled out, the potential fix might be completed around the end of January.

[lukaswittmann]

Thank you for your great work, looks good to me.

[lukaswittmann]

I renamed the file where the ceh numerical gradients are written to for consistency reason.

[Albkat]

I added the CLI argument description:

--ceh grad [REAL]

to calculate numerical gradients (with an optional step size specification).