Modernize project structure

This MR aims to modernize the HiggsDNA project strcuture. It used only a pyproject.toml for configuration. The scripts folder have been moved inside the higgs_dna folder with the source code and shell executables will be provided. The shell executables are for example: merge_parquet.py, prepare_output_file.py, ... Example usage: prepare_output_file.py --input=../somefolder/NTuples --merge They also work as python scripts still (python ../HiggsDNA/higgs_dna/scripts/postprocessing/prepare_output_file.py --input=NTuples/ --merge).

Such a structure allows HiggsDNA to be installed both in editable mode (pip install -e .) and as a package (pip install .). This allows users to just install HiggsDNA from gitlab (any branch/fork) without the need to clone the repo if they don't want to and all the required scripts are present as command-line executables. At the same time, this allows us to provide apptainer images with HiggsDNA pre-installed and no need to ever clone HiggsDNA within the apptainer.